handle git branches in target path

This change adds handling for the magic path element GIT_BRANCH to the remote path. It will be re-evaluated with each sync operation. Meaning if you change branches while synchrotron is running, it will sync the NEW branch to it's own dir on the server, and not crush the one you had previously been syncing.
Update synchrotron.rb

- Update based on review feedback from @yaypie
- slight tweak to the git branch grabbing regexp
Merge pull request #1 from cabbey/master

Add support for GIT branch specific remote paths