Add unicode type detection for consumer key/secret

The hmac module won't accept unicode objects so cast them before using
them if they're present.

I have some python-spidermonkey code, which uses unicode objects exclusively (much like py3), so I needed some code in tweepy to automatically handle incoming unicode consumer key/secrets and cast them as bytes so the hmac module doesn't throw a fit. This will be necessary for py3 support as well (I think).