Provide detailed usage examples

[bgandon]

This is #31 that has been successfully rebased on master branch.

Original description from @drnic:

This PR includes a full example (now in a runnable script) to target, authenticate using password grant, look up /token_keys to get public signing keys, decode an access token using the signing keys, and to call the UAA API /userinfo endpoint as the user.

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Benjamin Gandon (ffc4921, fbcc851)

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this:

https://www.pivotaltracker.com/story/show/181025356

The labels on this github issue will be updated when the story is started.