Add shebang to update scripts and fix Xcode references
Rename GTReference `-type` to `-referenceType`

They were incorrectly converted to git object types. This is how rugged does it.
Make GTObject more classcluster-like.
Fixes after review.
Make `-target` saner

by making it return a GTObject if it's a direct reference or another GTReference if it's symbolic (a.k.a it does only one level of resolution) and `-resolvedTarget` & `-resolvedReference` which respectively return the GTObject behind a chain of reference, or the last direct reference in a chain.
This one can be pretty-printed.
Rename `-target` so the compiler complains.
Code conventions
Implement <NSCopying> for GTOID.
Add init method to initialize GTOID with a SHA C String and use this in the initWithSHA: method.
Add convenience methods to create GTOID objects.
Fix style issues.
Add unit test for -initWithSHACString:
Fix more style issues.
Add documentation and fix style issues for the convenience initializers.
Remove unused reference type.
GTEnumerator; don't convert an OID to a SHA just to get an OID

Saves ~15% runtime when walking a large tree (linux kernel)

Improvement for libgit2/objective-git#187
Merge pull request #207 from rowanj/walkPerformance

GTEnumerator; don't convert an OID to a SHA just to get an OID
Add initializers that can return a NSError instance on error.
Replace git_oid by GTOID in GTRepository.