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!
It's quite backwards to have createScopedRequired and createScoped.

Here, you initiate a client and ask for the initiated client to if it is a valid client or not. You shouldn't initiate an auth client with missing required arguments to start with.

If the intention is to give user a utility to check whether they need to provide credentials or not, it should be a utiliy function that is separate from the existing auth clients.