Add support for modular build structure. (#79)

* Make the library modular usable.

* Switch to library requirements instead of source. As source puts extra source in install targets.

* Add requires-b2 check to top-level build file.

* Bump B2 require to 5.2

* Change all <source> references to <library>.

* Update copyright dates.

* Move inter-lib dependencies to a project variable and into the build targets.

* Update build deps.

test/Jamfile.v2

@@ -1,5 +1,6 @@
 project
     :   requirements
+        <library>/boost/mpl//boost_mpl
         <warnings>extra
         <toolset>clang:<cxxflags>-Wno-variadic-macros
         <toolset>gcc:<cxxflags>-Wshadow
@@ -49,7 +50,8 @@ compile find.cpp ;
 compile find_if.cpp ;
 compile fold.cpp ;
 run for_each.cpp
-  : : : <cxxstd>98:<build>no <cxxstd>03:<build>no ;
+  : : : <cxxstd>98:<build>no <cxxstd>03:<build>no
+    <library>/boost/bind//boost_bind ;
 compile front.cpp ;
 compile get_tag_def.cpp ;
 compile has_xxx.cpp ;