Add callback uri for revoked access tokens, fix bug in generator

* WePay added a new optional parameter in `/oauth2/token` called `callback_uri` for receiving IPNs for revoked access tokens for a user. Add this to `get_access_token` method.
* Remove the `belongs_to` associations in `WepayCheckoutRecord`, as those are not required by default and can be customized.
* Fix a bug in the install generator where `WepayCheckoutRecord` is not copied to the correct directory.
* Convert README from .rdoc to .md and add an example of using the gem to authorize a user.