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. 
Should this resource be responsible for installing Git? Typically, you would use a separate resource (perhaps `Package`) to install software, and have this resource run after that one by way of a `DependsOn` attribute.

If someone calls this resource when `IsGitInstalled` is false, this resource can just throw an error.