Changed return type of DiffCount. 

Now gives more detail about elements changed.

Also, added isClean method to WorkingTree and Index classes, for faster checking.

The output of some cli commands has been adapted to use this extra information about changes.

This changes fix issues when adding empty feature type. The count of unstaged elements was reported as zero, ignoring the added tree if it was empty

This PR replaces https://github.com/opengeo/GeoGit/pull/254