Python 3 compatibility

[pksohn]

This PR includes several code changes to be compatible with Python 3, and changes the Travis build to work with Python 3.5 and 3.6. Addresses #29. This is necessarily in parallel with changes to urbansim_parcels, pandana, osmnet, and urbansim; running a simulation using Python 3 using this new codebase also requires Python 3 compatibility with several of the above. The Travis build currently grabs the py3-compatible branches of the above, so that will have to be updated over time.

Code compatibility changes

• Print statements changed to print functions, along with from __future__ import print_function
• Implicit relative imports changed to explicit absolute imports.
• developer.py module renamed to develop.py to avoid namespace conflict related to absolute imports.
• A couple changes to dict: iteritems() method replaced with items() (same behavior), dict.keys().sort() changed to list(dict.keys()).sort().
• Change to file io (relevant in writing yaml files in test modules)
• True division rather than floor division, with from __future__ import division in python 2. I manually checked all of the division operations against test data and they return the same arrays either way.

Travis build changes

• Now builds on 3 versions of Python: 2.7, 3.5, 3.6
• Conditional installation of pandana from pip (2.7) or github on python3-support branch (along with similar installation of osmnet).
• Installation of urbansim_parcels from python3 branch
• Installation of urbansim from pksohn/python3 fork/branch

Again, we'll just have to keep track of where the Travis builds are pulling code from. It should get much simpler as we merge changes into master branches.