Config-loading tests

[the-overengineer]

Adds tests for config loading. Tests these features:

• Is config loaded from the default config.yml
• Is config loaded from a local stormpath.yml
• Is config loaded from a local stormpath.json
• Is config loaded from the environment
• Is config loaded from the config object
• Is configuration overridden in the proper order
• Are errors thrown if the id/secret/app_href are missing

Needs a tiny bit more work, however - when testing the application href missing line, the application is complaining that the ID/Secret are incorrect, after being generated for a new account created for that very application. Spent some time trying to track it, but running into a wall. Maybe a fresh pair of eyes would be helpful.

Fixes #286