PYTHON-346 - Use a tuple for decoder options.

This is just a cleanup of the existing decoder. I tried
using a namedtuple but that imposed up to a 17% perf hit
(a regular tuple imposed no measureable perf hit). We may
be able to avoid that problem with a new API that accepts
decoder options in a specific class instead of creating the
instance in the decoder itself.
PYTHON-346 - Eliminate _get_int unsigned option

This cleans up a few pylint complaints and provides a
small perf improvement.