Add a GeoGit contrib package.

**What does this PR do?**
This PR adds the GeoGit integration from the ROGUE project to GeoNode as an optional module.  The two integration points included in the PR are 1) a GeoGit tab on the layer detail page that is displayed when the current layer is a stored in a  GeoGit repository and 2) new geogit link objects that are created in the geoserver post save signal.

**Enabling this functionality:**
To enable this functionality, simply add `geonode.contrib.geogit` to the `INSTALLED_APPS` tuple in your `settings.py.`  You will also need to have a layer that is stored in a geogit repository.

**Screenshots:**
![screen shot 2014-07-11 at 1 51 39 pm](https://cloud.githubusercontent.com/assets/1141646/3557008/ac415648-092a-11e4-8c09-5541417722a0.png)

**To Do:**
We are also planning to submit a PR which modifies the geoserver build to include GeoGit.