Python 3 compatibility: don't use tuple unpacking in lambdas.

Python 3 got rid of tuple unpacking in function arguments, so we have to use traditional indexing instead.

(I'm considering changing ttftree.CompoundMeasure to accept a custom tuple class to use; then I could pass in a custom namedtuple with "index" and "key" attributes.)

We're now syntax compatible with Python 3, but a few more things (such as calls to dict.iteritems) need to be changed before we can do anything useful. I'll be using six for that.
Require six
Use items instead of iteritems.

This gains us Python 3 compatibility. I still need to benchmark this to see how much of a performance hit it results in.