Tweaks to Variable and Dataset repr
Add DataView and indexing
Orthogonal indexing and cached indices
Added utils.num2datetimeindex and collapse operators
Simpler Dataset.views + working non-trivial indices + aggregated_by
Refactored backends for more consistency/flexibility
Added virtual variables
Added utils.Frozen
Moved data.py to dataset.py
Fixed iterator and removed broken methods
Renamed views & loc_views to indexed_by and labeled_by
More flexible __getitem__ and __setitem__
Patched in more numpy methods
Faster aggregate

Now O(n) instead of O(n^2), even though we do have to iterate through every
element of an array in Python (gasp!).
More array interface tests for ufuncs
Fixed DataView.from_stack
Initial docs with Sphinx
`to_dataframe` method implements pandas.DataFrame export

Refactored the `broadcast_variables` function and fixed a bug in
`variable.transpose` (see the new test case).
New README.md
Fixed intersection

Added better test that uses string labels instead of just integers.