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.
I left it in to provide an easy way to run the tests with implicit conversions disabled, but left it commented because we don't usually want to do that.

I was considering checking an environment variable for it, but doing that properly (only setting the default encoding around test cases rather than at import time where it can leak out of tests, etc.) is a bunch of extra work. I'm happy to do that if you're okay with the rest of the branch.