WIP, start implementing incremental
Merge branch 'master' into p4_master_build_step

Conflicts:
	master/buildbot/test/unit/test_steps_source_p4.py
add do_sync flag for experimenting robocopy support
added new slave command 'MirrorDirectory'

On posix system, it uses rysnc
On Windows system, it uses robocopy (robocopy is builtin after Windows Vista, for Windows prior to Vista, you may get it from Windows Server Resource Kit)
Merge branch 'master' of https://github.com/jonnyyu/buildbot