Initial import of the geogit-geoserver plugin.

The geogit-geoserver plugin enables the configuration of geogit repositories
as GeoServer data stores, and geogit feature type trees as GeoServer layers.

It also exposes the geogit web api at <geoserver base address>/geogit (e.g.
http://localhost:8080/geoserver/geogit)

Or to a specific repository at <geoserver base address/geogit/<repository>,
where <repository> is given by the <workspace:store> name as configured
in GeoServer as a data store.