updated vectorstore.py's write_vectors_to_disk so that the features file only contains features with a non-zero count
updated thesaurus_loader.py and unit tests to deal with multi-words entries in a thesaurus
Integrated two bits of functionality together:
1) Build Byblo thesaurus in two steps with a pause in the middle to include multi-word expressions
2) Calculate composed vectors for multi-word expressions (using a set of dummy composers)

Everything runs but all unigram entries get zero vectors, a result of my messing with them
fixed minor bug where the n-gram vectors produced by this software had the wrong line ending (\r\n) and thus appeared to have empty lines
Fixed a few minor bugs in building phrasal thesauri with Byblo:
- removed csv module in vectorstore.py because it was producing windows line endings
- added an aggressive_lowercasing option to Thesaurus, which allows usto turn of lowercasing when feature files are being read.

 The entire thing now runs end-to-end, producing phrasal thesauri