Decoupling packages

[callmehiphop]

RE: https://github.com/GoogleCloudPlatform/gcloud-node/pull/1440/files#r73738579

There are a number of instances within our code where we (sort of) couple packages together. For example, in the NL package we offer optional support for annotating GCS objects. I think it would be great if we could remove the actual storage module dependency while still supporting this feature.

Possible solutions could include:

• Using optionalDependencies
• Check for inheritance of ServiceObject
• Check the constructor name

[stephenplusplus]

This will fix issues like #1516.