Added model needed for Local Repo API

Added a model package for data model. Nothing has been tested yet. Will
test within Local Repo API, which is next up on my list of things to
do. The following classes have been added or updated to meet
coding/documenting spec, which is still a work in progress:
- Branch
- CommitNode
- GitObject
- GitUser
- Sha1

Please see documentation for detailed information on each model object.