Set up basic Travis builds on 2.6 and 3.3.

Closes #135

This is a start. It tests on 2.7 and 3.3 with mpl 1.3, jinja2 2.7.2, as well nose and pandas (0.13.1) which are needed for the tests.

For now, I'm running the contents of `mpld3/tests/`.

Later, it may also be useful to also generate the `test_plots/` and watch for exceptions, even though we can't assess their appearance in Travis. It will take some cleverness to keep those scripts from freezing the build process, but that shouldn't be difficult.

I added a build badge to the README. Also, if we are not testing in py2.6 (as @jakevdp mentioned in some other issue) then we shouldn't advertise support for it, so I removed that from the README. I can take out that commit if you disagree. On that subject, I *will* add a build for 3.4 in the next day or so. I ran into a conda problem with that, so I will make it a separate PR once it is settled -- let's get this out there.