Infrastructure

A bit of cleanup to the setup.py and the repo layout (moved C++ source files into src/ directory). Big changes are that we are now on Travis (https://travis-ci.org/synthicity/urbanaccess) and coveralls (https://coveralls.io/r/synthicity/urbanaccess). And I add a skeleton sphinx setup in the docs/ directory.

Still to do:

- [ ] add build/coverage badges to the README
- [ ] change name to pandana everywhere