* Break circular dependency between core and util
Created a new :common project and moved a minimum
number of classes to break the circular dependency
between the two projects. This gets rid of the
gradle lint dependency warnings.
Also separated api classes and testing helpers into
separate source sets in :common so that testing
classes may be restricted to test configurations.
Replace deprecated GoogleCredential with new lib
This PR also introduced a CredentialsBundle class to carry
HttpTransport and JsonFactory object which are needed by
most of the GCP library to instantiate client.
