Python 3 fix
change default parameters for building ConceptNet
Merge with Luminoso version

Luminoso has been developing a version of assoc_space in parallel for a
while now, including bug fixes and improvements. We're merging that
version into the open-source assoc_space module.
Use the upper triangle of the matrix by default
Refactor how SparseEntryStorage is used; make room for merge_similar

The 'merged_with' method is now named 'merge_dissimilar', because there
will be a parallel 'merge_similar' method.
Add tests for LabelSet and SparseEntryStorage
more porting and refactoring; WIP on similar matrix merging
give up on combine_similar_eigenspaces; add methods

The combine_similar_eigenspaces technique turned out to work worse than
combine_dissimilar_eigenspaces, so it's been removed.

Also, added (and in many cases re-added) methods that ConceptNet's
assoc-building code depends on.
make the save directory if it doesn't exist
put back the function signature of combine_eigenspaces
Add some interactive pretty-printing features
begin working on documentation
fix a type-check and some docstrings