Port to Python 3.5

[tdsmith]

This branch is compatible with both Python 2.7 and Python 3.5.

I adopt the convention that the Protocol class must accept and emit Unicode strings and is responsible for converting objects to bytes.

The changes to the tests are mostly small compatibility fixes with a few exceptions:

• my redis is truncating floats in the sorting test differently than the test suite expects, so I'm skipping the test; I don't know if it's important to test the precision but this feels like something that Redis has changed
• test_rename_same_src_dest succeeds on my redis install; it looks like renaming to the same name is allowed, now

I propose deleting the rename_same_src_dest test and adopting less stringent tests for the floating point sort test.

Thanks!

[tdsmith]

Incidentally: the awesome test suite made this much easier!