Added Association-bidir

git-svn-id: http://svn.boost.org/svn/boost/sandbox/association@61857 b8fc166d-592f-0410-95f2-cb63ce0dd405
cleanup

git-svn-id: http://svn.boost.org/svn/boost/sandbox/association@65186 b8fc166d-592f-0410-95f2-cb63ce0dd405
cleanup

git-svn-id: http://svn.boost.org/svn/boost/sandbox/association@65187 b8fc166d-592f-0410-95f2-cb63ce0dd405
Association: document and modify a little bit the interface

git-svn-id: http://svn.boost.org/svn/boost/sandbox/association@78522 b8fc166d-592f-0410-95f2-cb63ce0dd405