Fix invalidate_token method

Leftover from refactoring into processTokenResponse()
invalidate_token() now works as advertised.

**Invalidate process (for the docs):**
Set consumer keys and call invalidate_token($token);
This will return token itself if correctly invalidated/revoked,
or will throw a twitter auth error (99) if credentials or token invalid.

and added basic curl error reporting, in preparation for #12