Cleanup of tests; package absolute path imports

Mainly, this sets things up so that imports (except those in the main directory) are absolute imports. So that affects storage and tests in particular. In the process, I also set up a tests/test_data/ directory, which has a PDB trajectory which we convert to our storage format in the test-package-level setup fixture (this way, we don't need to add our currently bulky .nc files to the test directory). Oh yeah, and to do that, I wrote a `Trajectory.from_mdtraj()` function (we could already get to mdtraj, but I didn't see ways to convert the other way yet).

Most importantly, this means that you should now be able to run the nosetests from the main directory as well as from the tests/ subdirectory, which made sense to fix before automated testing (#10). 

Also, I included a trivial test_always_pass file to the test. @jhprinz, you're planning to set up the travis testing, right? My suggestion is when first setting it up, have it call the tests as `nosetests test_always_pass`; this way any weirdness related to the *actual* tests won't matter, and if nose and travis are working correctly, the first version will say the test passed. Then just drop the `test_always_pass`, and it should run the real tests.