Implement <NSObject> methods for GTSubmodule
Optimize and test GTOID
Add -[GTOID description]
Clean up and test GTSignature
Implement <NSObject> methods for GTReference
Apply objc_returns_inner_pointer to all libgit2 objects
Merge pull request #194 from libgit2/add-nsobject-methods

Add <NSObject> methods, optimize GTOID, use objc_returns_inner_pointer attribute