Updated AuthorsFile.cs to allow bidirectional mapping between git author tuple and a tfs user id.
Extended GitCommit with a property to retrieve the git author name/email tuple.
Added author parameter to Checkin method.

- Added IWorkspace.Checkin overload that takes an author parameter
- Implemented the method in the VSx-Wrappers.
Added AuthorFilePath and Author fields to Checkin options.

Short-cut a ("a|authors" consistent with other uses) is already used by a|autorebase. Used "A|authors" instead.
Added authors to CommitSpecificCheckinOptionsFactory

- Added overloaded BuildCommitSpecificCheckinOptions that takes author parameter
- Added ProcessAuthors function that tries to map the git author to a tfs user id.
Rcheckin now passes the GitCommit instance to the checkin options factory.
TfsWorkspace now passes the author parameter to Checkin(...)
Identification of git users for git-tfs user mapping now only uses email address.

This was done to avoid mismatches when enforced Active Directory naming rules and developer's chosen name formatting did not match up.
Fixed code that failed test WorkspaceTests.Policy_failed_and_Force_with_an_OverrideReason.

- Updated test calls.
- Removed duplicate versions of Checkin and EvaluateCheckin without author.
Made BuildGitUserId public static

Clients can now transform git author name/email tuples into a lookup key.
Added test methods to AuthorsFileUnitTest

...to test and document the new behavior added for correct author attribution.
Refactored TryGetValue to FindAuthor

- Changed function signature as requested by sc68cal.
- Updated clients and tests.
Updated GitCommit.AuthorAndEmail property according to sc68cal's suggestion.
Merge pull request #336 from l3m/correct-authors

Correct author attribution