Added support for getting service by GUID

Added getService(UUID) to CloudFoundryClient / CloudControllerClient, and also added buildpack_version to Application.Staging
This constructor is used by clients to specify starting command, buildpack, etc when creating app metadata. Changing the method signature will break existing clients. You should add a new constructor with the additional parameter to maintain backward compatibility. 

Also, I don't see this new constructor being called when app metadata is retrieved. Am I missing that?