Adding Git Support

Hi Tim,

this adds git support to wadf. Aside from adding the driver class I had to make some changes to the main wadf class as it had some svn-specific code in there (and as a result I made some minor changes to the driver interface and the svn driver)

Once you checkout this code you can test it by deploying `git@github.com:kierate/wadf-git-test.git`

```
$ wadf-deploy kierate/wadf-git-test #works with "vc_base=git@github.com:" in your wadf config
```
or
```
$ wadf-deploy -d vc_base=git@github.com:kierate/ wadf-git-test #should work any time
```

Let me know if you have any questions on this.

Cheers,
Michal