Node.js usable auth

(Now with all jslint issues fixed.)

This pull request extends the auth portion of the client library to add the usable auth pattern for service accounts. This allows the user to create a few lines of code which will correctly set up a credential client no matter whether the code is running in GAE, GCE, a managed VM, or on a local dev machine.

See https://developers.google.com/accounts/docs/application-default-credentials for more information about usable auth.

See the examples/default_auth.js file for an example of how usable auth works in Node after this checkin.

All unit tests pass, in addition to many new tests to cover to new code.

Thanks!
You are right that this logically belongs on the JWT client. But this is intentionally placed onto the GoogleCredentials class for consistency with the other implementations of the usable auth feature. This is done so that users don't have to think about JWT vs Compute in most cases. They just use the GoogleCredentials class and they are happy.