WIP: Implementation of Git DB Commits api

Implementation includes:

- [x] `Get` request method plus unit tests.
- [x] Refactored `Tagger` to more generic `UserAction` 

_Is the refactoring ok? Or would you prefer multiple identical classes for each usage - `Tagger`|`Commiter`|`Author`)

Work outstanding:

- [ ] Implement `Create` method on `CommitsClient` with unit tests and integration tests