[CHEDDAR] Improvements to HttpClient to enable Oauth 2.0 grant flow closer to the rfc standard

- HttpClient has been improved to now request the address of the authentication server from the authorisation service correctly complying with steps taken in a successful the grant flow authentication.
- OAuthUserAgent interface has been updated to take the redirect uri of the authentication server.
Merge pull request #39 from clicktravel-james/master

[CHEDDAR] Improvements to HttpClient to enable Oauth 2.0 grant flow closer to the RFC