Distinguish between NULL and UNSET values in Prepared Statements

https://github.com/datastax/python-driver/pull/321

for test review:

Expanded unit tests to cover all bind paths for all supported protocol versions.

New integration test verifying encoding and expected semantics of UNSET.

These are on the unmerged feature branch.

+1

Left a few comments. Mostly nits. Other than the one validation question +1

input incorporated here