Add support for headers in first level sublibs

The sub-libraries that reside in the first level subdirectories in the
library git module can now have headers inside include/boost
subdirectories and 'b2 headers' will now link those headers into
the global tree.

The use of sublibs has been discussed in these threads:

    - http://thread.gmane.org/gmane.comp.lib.boost.devel/253920
    - http://thread.gmane.org/gmane.comp.lib.boost.devel/252054
    - http://thread.gmane.org/gmane.comp.lib.boost.devel/236737