Feature/password strength

This is the same as #51 with these additional changes:

* Squashed the original PR.
* Changed VCR to use Faraday instead of Webmock, in order to keep response bodies in plain text.
* Generated VCR cassettes for password strength against a test server, then sanitized. cPanel is currently returning numeric string values from their API on `get_password_strength`, so I updated the spec to allow this.