Switch the `lookup` methods to `id` to kill casts.
Add a convenience method to get all tags in the repository.

Fixes #27.
Merge branch 'master' into tag-enumeration
Changes per review + merge master.
Merge branch 'master' into kill-casts

Conflicts:
	Classes/GTBranch.m
	Classes/GTCommit.m
	Classes/GTEnumerator.m
	Classes/GTRepository.h
	Classes/GTRepository.m
	Classes/GTTag.m
	ObjectiveGitTests/GTBlobTest.m
	ObjectiveGitTests/GTCommitTest.m
Merge pull request #221 from tiennou/tag-enumeration

Add a convenience method to get all tags in the repository.
Merge pull request #215 from tiennou/kill-casts

Switch the `lookup` methods to `id` to kill casts.