significant reorganization

[poldrack]

hi - I have gone in and done some pretty substanatial reorganization. would be great to merge this into the main repo, but I will understand if you don't want to. major features:

• set up as a standard python package, with setup.py
• moved example script that runs the analysis into scripts/run_node2vec.py
• moved helper functions from that script into the main node2vec/node2vec.py
• wrote a couple of simple tests (in the tests/ directory)

I tried getting it working in python3 but something funky is going on, as the main test hangs and never completes, so I added a test in the setup.py to ensure that the user is using python 2.7. Will try to fix that later.

I am planning on using this to do some analyses of neuroimaging data, after our discussion with Jure yesterday. will add in those functions as they develop.