Now supports `__str__` and unicode topic strings.

Kafka will throw an error if given a Unicode topic string.  This fix
updates the constructors for `Producer` and `Consumer` to convert
Unicode strings to ASCII, and to support topic strings which are
actually objects implementing the `__str__` interface.
Merge pull request #3 from jirafe/handle-unicode-topics

Now supports `__str__` and unicode topic strings.