Client Credentials strategy for automated or user-less scripts

* https://developers.bootic.net/api/authentication/#client-credentials-grant
Make sure optional on_new_token block is called
typo
Strategy#root. Client knows nothing about default URLs.
Logger, api_root can be configured in BooticClient.configure block
Authorized strategy for web apps.

This flow starts with an existing access token.
Merge branch 'oauth_strategies'