Add testing framework.

[bdice]

Pythia currently lacks testing and continuous integration.

[bdice]

I have added continuous integration and one test via #4, #5, #6. Now we're ready to write more tests! @jdaaph plans to take this on during the hackathon. 👍

[klarh]

In case any trajectory files are needed, I'd recommend grabbing them from the bitbucket project's downloads (we should probably move those over to github at some point...) like the example notebooks do.

[bdice]

https://bitbucket.org/glotzer/pythia/downloads/

[csadorf]

Looks like this is resolved?

[bdice]

@csadorf The package is around 52% coverage - I would like to improve that (perhaps at a subsequent hackathon) before closing this issue.