Explicit unicode handling

I ran the test suite with the default encoding set to `'undefined'` (see `riakasaurus/tests/__init__.py` for that) and added explicit encoding and decoding to all the places that needed them.

There are still two tests that fail with encoding errors, but the implicit conversion happens in the bowels of Python's standard library (`xml.etree`, to be exact) and there's nothing we can do about it here.