Implement more extensive API

[Kennethtruyers]

See GitHub API for example

This is needed to be able to build a front-end application for GitDb

[Kennethtruyers]

First idea for API surface:

Git data:

• Blobs (get, create)
• Commits (get, create)
• References (get, get all, create, update, delete)
• Tags (get, create)
• Trees (get, get recursively, create)

Repositories:

• Repos (create, delete)
• Branches (get all, get)
• Commits (list, get, get sha, compare)
• Merges (perform merge (rebase, FF, normal)