Re-implement "auth" module to be instantiable

[jugglinmike]

Currently, the "auth" module is implemented as a singleton. This makes it difficult to use in contexts where the same process exposes multiple OAuth endpoints. My use case is a versioned API that defines dedicated authentication routes for each version.

In the short-term, I am re-installing this module in each of my project's API-version-specific directories. Another approach (for anyone else running up against this issue) would be to remove this project's files from require.cache in each place it is used.

Ideally, I would like to be able to use the auth object in the same way as the client object. To demonstrate, here's how that would look for an example from the project's readme file:

- github.auth.config({
+ var auth = github.auth({
    username: 'pksunkara',
   password: 'password'
+ });
- }).login(['user', 'repo', 'gist'], function (err, id, token) {
+ auth.login(['user', 'repo', 'gist'], function (err, id, token) {
   console.log(id, token);
  });