Remove #import GTOID+Private

Whoops
Remove `GTBlameOptionsMinimumMatchCharacters` in .m
Fix crash when deference `git_oid`.
GTBlameHunk: Remove unimplemented properties.
Document classes.
Use `SHA` instead of creating an OID
GTBlameHunk: Switch to `NSRange` property.

Instead of `lineCount` and `finalStartLineNumber` properties.
We don't use the `oldOID` variable anywhere so remove it.
Better testing of the options parameter.

Specifically, `GTBlameOptionsFirstLine` and `GTBlameOptionsLastLine`.
Better test description.
GTBlame: Subclass `isEqual:` and `hash`.
Better tests for `GTBlameOptionsNewestCommitOID`.