Implement SpecTemplate.sources

To get the ordered list of all Sources within them spec template.

related to xenserver/planex#53
Rename GitHubSource to GitSource

To prepare this source to be relaxed from github constraints.

related to xenserver/planex#53
GitSource: relax path restrictions

GitSource can now clone deeper hierarchies.

related to xenserver/planex#53
planex-clone doesn't deal with branches

The clone command should not be responsible for checking out the branch,
it should just clone the repository.

related to xenserver/planex#53