Add Composer dependency on an OAuth2 plugin for Guzzle 4. Need to test it out.
Add an option for specifying an access token when building the client. This allows users to bypass using `AuthService` because it assumes they are providing the Citrix OAuth implementation themselves. Remove the Guzzle OAuth2 plugin dependency.
Add throws declaration for Guzzle's `RequestException` to `Client#sendRequest`. This will account for most errors  produced by the API.
Merge branch 'oauth' into develop