Renamed include/ folder to gsl/ to make including the library consistent

whether using it from the development folder, from the installation
folder or from being copied into a project. 

    #include <gsl/gsl.h>

Updated headers/tests/instructions/cmake build accordingly

This PR should address https://github.com/Microsoft/GSL/issues/277 (less
the renaming of gsl itself)

tests/CMakeLists.txt

@@ -9,7 +9,7 @@ endif()
 add_subdirectory(unittest-cpp)
 
 include_directories(
-    ../include
+    ..
     ./unittest-cpp
 )
 
@@ -33,7 +33,7 @@ else()
 endif()
 
 function(add_gsl_test name)
-    add_executable(${name} ${name}.cpp ../include/gsl.h ../include/gsl_assert.h ../include/gsl_util.h ../include/multi_span.h ../include/span.h ../include/string_span.h)
+    add_executable(${name} ${name}.cpp ../gsl/gsl.h ../gsl/gsl_assert.h ../gsl/gsl_util.h ../gsl/multi_span.h ../gsl/span.h ../gsl/string_span.h)
     target_link_libraries(${name} UnitTest++)
     install(TARGETS ${name}
         RUNTIME DESTINATION bin

tests/assertion_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h> 
-#include <gsl.h>
+#include <gsl/gsl.h>
 
 using namespace gsl;
 

tests/at_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h> 
-#include <gsl.h>
+#include <gsl/gsl.h>
 #include <vector>
 #include <initializer_list>
 

tests/bounds_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h> 
-#include <multi_span.h>
+#include <gsl/multi_span.h>
 #include <vector>
 
 using namespace std;

tests/byte_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h>
-#include <gsl_byte.h>
+#include <gsl/gsl_byte.h>
 
 #include <iostream>
 #include <list>

tests/multi_span_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h>
-#include <multi_span.h>
+#include <gsl/multi_span.h>
 
 #include <iostream>
 #include <list>

tests/notnull_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h> 
-#include <gsl.h>
+#include <gsl/gsl.h>
 #include <vector>
 
 using namespace gsl;

tests/owner_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h> 
-#include <gsl.h>
+#include <gsl/gsl.h>
 #include <functional>
 
 using namespace gsl;

tests/span_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h>
-#include <span.h>
+#include <gsl/span.h>
 
 #include <iostream>
 #include <list>

tests/strided_span_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h> 
-#include <multi_span.h>
+#include <gsl/multi_span.h>
 
 #include <string>
 #include <vector>

tests/string_span_tests.cpp

@@ -16,7 +16,7 @@
 
 #include <UnitTest++/UnitTest++.h>
 #include <cstdlib>
-#include <string_span.h>
+#include <gsl/string_span.h>
 #include <vector>
 
 using namespace std;

tests/utils_tests.cpp

@@ -15,7 +15,7 @@
 ///////////////////////////////////////////////////////////////////////////////
 
 #include <UnitTest++/UnitTest++.h>
-#include <gsl.h>
+#include <gsl/gsl.h>
 #include <functional>
 
 using namespace gsl;