Make SubmoduleUpdateCommand an interface

To do the way that RemoteGitImpl is done, we need the SubmoduleUpdateCommand
to be an interface and not an abstract class.
Merge pull request #127 from dbroady1/SubmoduleUpdateCommandInterface

Make SubmoduleUpdateCommand an interface
[maven-release-plugin] prepare release git-client-1.8.0
[maven-release-plugin] prepare for next development iteration
Remove useless println calls from GitAPITestCase