PR 48 from dooblem/bsync: New modes: mirror and backup, while maintaining the bsync snaphots

[mikebentley15]

Original pull request on dooblem/bsync

I made three commits:

1. bsync didn't work in Windows with the only rsync I've found (cwRsync). cwRsync doesn't recognise disk-letter colons, Bsync had similar problems with Windows. First commit fixes this.
2. bsync required that all conflicts were resolved immediately. In batch mode, bsync stopped on the first conflict. The second commit allows correct handling of unresolved conflicts - they are left for later bsync runs. This is also required for the third commit.
3. Third commit implements new uni-directional synchronization modes: backup and mirror (backup doesn't synchronize deletes). These modes correctly update bsync snaphots. Execution of bi-directional sync and uni-directional mirror and backup can be mixed without breaking the snapshots. Second commit is required.

I also added python unit tests of bsync of all tree modes.

Note: I tested that only locally in Windows, though I tried to keep it working in posix/ssh.