Add method for creating a tree in a repository
Add a model object for git refs
Add a model object for git commits
Add methods for fetching and updating refs
Merge branch 'split-client-categories' into more-git-data

Conflicts:
	OctoKit.xcodeproj/project.pbxproj
	OctoKit/OCTClient.h
	OctoKit/OCTClient.m
	OctoKit/OctoKit.h
Add a method for creating a blob
Commit fetching
Commit creation
Fix type of `treeSHA` parameter
Ignore cached data when fetching refs

Apparently these get cached for 60s, which can result in stale data
when performing git operations.
Assume that OCTTreeEntry serialization succeeds

Matches OCTClient+Gists and OCTClient+Keys.
Sample JSON for commit and ref fetching
Test OCTCommit parsing
Test OCTRef parsing
Merge pull request #128 from octokit/more-git-data

Support more kinds of Git data