Correct author attribution for RCheckin Quick

I extended the code for correct author attribution (#336) and refactored some duplicate existing code.

- Implemented correct author attribution for RCheckin Quick Mode.

- Split up the large PerformRCheckin function into 3 parts:
  - PerformRCheckin
  - _PerformRCheckinQuick
  - _PerformRCheckin

- Created RCheckinCommit class to encapsulate identical code between the two _RCheckinCommit functions
  - Before, that code was simply duplicated in the function.


**Open Questions**

[RCheckinCommit](https://github.com/l3m/git-tfs/blob/quick-authors/GitTfs/Commands/Rcheckin.cs#L189) is in the RCheckin.cs file. I am not sure where to put it; can it stay, or in Utils? What would you suggest? 

There's still some ugliness in [Util/CommitSpecificCheckinOptionsFactory](https://github.com/git-tfs/git-tfs/blob/master/GitTfs/Util/CommitSpecificCheckinOptionsFactory.cs#L161). I directly create and load the authors file. It's the only 'new AuthorsFile' outside of unit tests, I assume this is because of your DI framework. How can I solve this better?