Use wheels to manage Python builds

@psung, please review.

The impetus for this was inability to run the test suite on any system where test dependencies were present in versions different from those we wanted to specify. The solution for that is to have setup.py look at an environment variable and include the test dependencies in install_requires.

I rolled in a few more changes here, including a simplification of the build process using wheels, which makes the build complete on all of Python 2.7, 3.3, and pypy inside or outside of a virtualenv. (https://travis-ci.org/dnanexus/dx-toolkit)