Remove DataArrays

This removes all of the code for DataArrays from DataFrames. I'm merging it even though it's slightly broken because we need to get this done sooner rather than later.

There are two bugs:

* The I/O code is broken. This is also true on master, but that's no loss to us.
* The `join()` code is broken. This problem is a little bit subtle, but should be possible to track down.