[WIP] Added directory

Not sure if we need something like this, but IMHO it might be nice to push some of the `featurize_all` functionality into a class.  I still haven't worked out how to deal with chunking and stride, as it's a little less clear about how they will work in this kind of framework.  Here's some example code:

```
import mixtape.directory
import mixtape.featurizer, mixtape.tica

directory = mixtape.directory.Directory("./system.subset.pdb", "./Trajectories/")
directory.cache()

atom_pairs = np.loadtxt("./SlowAtomPairs.dat", 'int')
featurizer = mixtape.featurizer.AtomPairsFeaturizer(atom_pairs, directory.trj0)
data, indices, fns = directory.featurize_all(featurizer)
```