Consider rename XXXService to XXX

[aozarov]

Currently the service naming style in gcloud-java is as follow:

API:
XXXServiceFactory
XXXService
XXXServiceOptions
XXXServiceException

SPI:
XXXRpc
XXXRpcFactory

where XXX represent the service such as Datastore, Storage,...

I think it would be nicer if we drop the "Service" part and make the API names shorter, such as:

DatastoreFactory, Datastore, DatastoreOptions, DatastoreException
StorageFactory, Storage, StorageOptions, StorageException

[aozarov]

Done.