Replace git_oid by GTOID in GTRepository.
Use GTOID in NSString+Git category.
Replace git_oid with GTOID in GTObject and GTObjectDatabase.
Merge branch 'master' into use_gtoid
Use GTOID in GTEnumerator.
Merge branch 'refs/heads/master' into use_gtoid
Convert git_oid to GTOID before using it in lookupObjectByOid:error:
Add containsObjectWithOID: method and implement containsObjectWithSha:error: using this method.
Add addEntryWithOID:... to GTTreeBuilder and implement addEntryWithSHA:... in terms of this.
Use GTOID class to parse SHA strings.
Add +OIDByCreatingCommitInRepository:... to GTCommit and use that to implement +shaByCreatingCommitInRepository:...
Add +OIDByCreatingTagInRepository:... to GTTag and implement shaByCreatingTagInRepository:... with that method.
Add OID property to GTTreeEntry and use it to implement the hash and sha methods.
Add the OID property to GTOdbObject
Fix some style and whitespace issues.
Rename GTObject oid to OID
Remove methods to convert between NSString and git_oid from NSString+Git category and use GTOID class instead.
Fix more whitespace issues in docs.
Insert missing newline
Merge pull request #211 from 5sw/use_gtoid

Use GTOID instead of SHA strings or git_oid structs