Created a DSC Resource for keeping Git repository in sync

Written this to keep a local git repository in sync with a remote repository. Planning to use for dev/test environment. 
Great!

If I might make a suggestion on the `cGitPull` resource, add an optional parameter specifying the path to git.exe.  If no value is passed for that parameter, the resource can try to find it on the system path the way it does already, but users have the option to override that behavior and point to a copy of git that's in some other location.  This should give users total flexibility on how and where they want to install git, and still allow `cGitPull` to work either way.