Modularize the work-in-progress Boost.Sync as part of Boost.Thread for now

This is not the right long-term solution, but it's possible that the
long-term solution should be handled after we switch to Git.  I'll
check with the Boost list for details.
Merge pull request #1 from ryppl/master

Modularize the work-in-progress Boost.Sync as part of Boost.Thread for n...