MAINT: enable travis

Enable travis-ci. Runs the full test suite on Python 2.7, and the fast
test suite on Python 2.6 and Python 3.3.

The test suite run times are in the ballpark of:

- 10 min for the fast test suite on Python 2.x
- 20 min for the fast test suite on Python 3.x (installing Numpy is slow)
- 30 min for the full test suite on Python 2.x