NEW FEATURE: Added possibility to feed SHKRequest with ready-made NSMutableURLRequest. Handy for OAuth2 types of requests.
BUGFIXES: SHKImgur

- added refreshToken routines
- simplified implementation (now uses SHKRequest, removed status flags - I believe that they make the code unnecessarily complicated. They are the opposite of functional programming)
- SHKOAuthViewDelegate declaration separated to special file to avoid header dependency