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.

example/Jamfile.v2

@@ -7,9 +7,11 @@
 
 import testing ;
 
+project : requirements <library>/boost/mpl//boost_mpl ;
+
 test-suite "mpl-examples"
    :    [ run     inherit_linearly.cpp ]
         [ compile integer.cpp ]
-        [ run     tuple_from_list.cpp ]
+        [ run     tuple_from_list.cpp : : : <library>/boost/tuple//boost_tuple ]
    ;