Make handling custom callbacks easier.
Make handling custom callbacks easier.

Note that Twitter recommend explicitly setting the callback despite the
default callback being set in the application settings at the following
URL:

  http://dev.twitter.com/pages/auth

See the "OAuth at Twitter" section.

Changes made:

- Added EpiOAuth::setCallback() for ease.
- This callback is then added to params for EpiOAuth::getRequestToken().

jmathai/twitter-async,ibobo/twitter-async,ibobo/twitter-async,jmathai/twitter-async,phansys/twitter-async,phansys/twitter-async