Add method to check if auth is working

We have `hasRegistrationKeys()` and `hasPermanentKeys()` which just check if the TentApp object has keys at all.  Should have another method which checks if the keys work.

Call it `verifyAuthKeys()`

For example, this could be done by doing a `getFollowers()` and checking if the auth fails in the process.  This will be easier once there is better error handling throughout python-tent-client.
