Don't require an options object for Service instantiations

[stephenplusplus]

With ADC auth, it's possible users don't need to provide any options object to a Service instantiation. That currently causes this line to throw:

https://github.com/GoogleCloudPlatform/google-cloud-node/blob/c98acd27ad8718a2c78bffa9b0f253cc334334b9/packages/common/src/service.js#L50

We should default options to an empty object.