Add GitInit for making new local repos

Over at MovingBlocks/Terasology we're playing with a fancy multi-repo Git setup and I've prepared some initial Gradle magic in a test organization. Was missing a simple way to initialize new local repos that would flow well with everything else and got this new task to work :-)

I tested it locally after building and deploying to our Artifactory - http://artifactory.movingblocks.net/artifactory/webapp/browserepo.html?22&pathId=libs-release-local:org/ajoberstar/gradle-git/0.6.2/gradle-git-0.6.2.jar

The GitInit task works fine, but my dependency-fu is failing me so my version can't do much of anything else as it fails to find the jcraft stuff from 0.6 for me - might be because I started with a 0.5 pom or something. Fun with caching, perhaps.

I suspect when built properly it'll work for the other stuff too. Or even tested as-is on a different machine. So I thought I'd send it this way :-)

For local build convenience I added:

ext {
    // Defaults so this script will run even without credentials available (but of course uploadArchives will fail if so)
    sonatypeUserName = 'none'
    sonatypePassword = 'none'
}

Was considering adding that in a separate PR but then versioning fun might happen ;-)

So FYI if you would be interested in adding that sometime, I'd like to think passing in the properties would override those.

I based this on GitClone and if it works out I might later try to sort out a way to automatically add a remote ref too.

Thanks for the great plugin!