pyndexter enhancements

Now that 0.2 is finally out the door, what next?

Firstly, making query term negation work in the default indexer. Not having this is suboptimal. My initial thought on how to solve this will be to create a set class with support for lazily evaluated complements.

Next up, more extensive unit tests. Specifically, I want to test the UTF support of all the indexer adapters.