[REST API] Updated the branch API to be a bit more REST compatible

[EricWittmann]

The API for branches has been updated a bit. The API now looks like this:

Endpoint	Operation	Description
/groups/:groupId/artifacts/:artifactId/branches	GET	List all artifact branches.
/groups/:groupId/artifacts/:artifactId/branches	POST	Create a new branch, can create an empty branch or a branch with some versions.
/groups/:groupId/artifacts/:artifactId/branches/:branchId	GET	Get branch metadata (description, createdOn, etc)
/groups/:groupId/artifacts/:artifactId/branches/:branchId	PUT	Update branch metadata (description, etc).
/groups/:groupId/artifacts/:artifactId/branches/:branchId	DELETE	Delete the branch.
/groups/:groupId/artifacts/:artifactId/branches/:branchId/versions	GET	List all versions in the branch.
/groups/:groupId/artifacts/:artifactId/branches/:branchId/versions	PUT	Fully update the list of versions in the branch, replacing the previous list (can be used to re-order).
/groups/:groupId/artifacts/:artifactId/branches/:branchId/versions	POST	Add a new version to the branch.