Minor cleanup
Simplified _build_crossvalidation_iterator and updated tests
Caching more

Previously only the call to tokenizer.tokenize was cached. Now the
dataset won't be read at all if a tokenized version of it is available
in the joblib cache
Doing crossvalidation serially in a single process

The overhead of serialising data to worker processes is too great for
it to be worth it. It might be possible to reduce that overhead, but it
is easier to run in a single process. Experiments are already quite
fast, and being single core I can run more of them at a time.

See https://www.pivotaltracker.com/story/show/76271838
Added code to cache all labelled corpora to joblib

Speeds up tokenizer and prevents conflicts between multiple experiments
trying to cache to the same location concurrently
Fixed issue with multiple "lowercase" parameter in conf file

This was left over in test conf file and isn't present in the files
I use to run experiments at the moment