Testing

In the process of writing my own unit tests for the `d()` and `d2()` functions, I updated the module testing to use the standard library `unittests` module, and set `.travis.yml` to test on Python 2.6, 2.7, 3.2, 3.3, and 3.4. There were a couple small things this found that needed changing:

  - The syntax `'{:}'.format(x)` needs to be `{0:}.format(x)` for it to work in Python 2.6
  - For making documentation, something about Pygments doesn't work in Python 2.6, so I disabled that in the `.travis.yml` file just for Python 2.6.

I got a bit carried away yak-shaving, but I think the result was a good one: this branch of `ad` now runs on 2.6, 2.7, 3.2, 3.3, and 3.4, unit tests are in standard `unittests` format (and thus can be used with something like `coverage`), and the module passes all unit tests with or without `numpy`.