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!
This still looks like the instructions would not work and are not the right ones. We want people to download the the JSON key, not the P12. Once that is done, pointing the environment variable directly to that downloaded is all that should be needed. This should not work on a .pem file on its own, because it is expecting JSon that contains the pem as one of its fields.