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/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;