Clarify documentation of `originalCommitOID`.
Fix `GTBlame` and `GTBlameHunk` visibility for iOS targets.
Expose all of `git_blame_options` as a `NSDictionary`.

- Adds the necessary GTBlameOptions* keys
- Because `git_blame_options.oldest_commit ` takes a `git_oid` instead
of `const git_oid *` as all other libgit2 APIs use; add a
`GTOID+Private` file.
Add tests for the new options keys.

Blame support is not fully implemented in libgit2 as such I tried to
write test that will fail when those features are implemented (as in
the case of `GTBlameOptionsTrackCopiesAnyCommitCopies`)  or I just
plain didn’t test them (As in the case of
`GTBlameOptionsMinimumMatchCharacters`).