Created branch to extend PropertyMap:
add a readable functor map that calls a given functor or function upon lookup.
Ideas from: http://comments.gmane.org/gmane.comp.lib.boost.user/54397
and: http://comments.gmane.org/gmane.comp.lib.boost.user/73358

svn path=/sandbox-branches/birbacher/propertymap-functormap/boost/is_placeholder.hpp; revision=77438