WIP: Added SpatialMeta class and tests.

First step in integrating dcmstack functionality into nibabel. Currently this is just the  class that will store the meta data associated with an image. Compared to dcmstack, this approach allows for an arbitrary number of image dimensions and a more comprehensive set of classifications.

I still need to implement the three hardest methods (simplify, get_subset, and from_sequence). However I figured it would be good to get the ball rolling and solicit feedback on the API.

Sorry this took so long to get going!