This change adds the following new resource implementations:

- RepositoryItemResource

- RepositoryListResource

- RepositoryBranchItemResource

- RepositoryBranchListResource

- RepositoryCommitItemResource

- RepositoryCommitListResource

- RepositoryGroupItemResource

- RepositoryGroupListResource

- RepositoryInfoResource

- RepositoryUserItemResource

- RepositoryUserListResource
This involves one update to our base ListResource implementation (and

various related plumbing) to allow subclasses to force a specific class

type to be used for items. In the case of repository branches and

commits, these are list resources that do not have an associated item

endpoint or mimetype, and so the basic _wrap_field() and create

functionality was returning a plain ItemResource. For this case,

ListResource subclasses can now set _item_resource_type to force a

specific item resource subclass instead.

Ran unit tests.
Used the new resources from a test script and saw that everything

  worked as expected.