Fix tondarray and fromndarray.

Don't use IPython's scatter and gather anymore.

These are pretty slow and should be sped up in the future. This can be
done by speeding up __getitem__ and __setitem__.
Allow distarray ufuncs to take casting kwarg.
Correct tests for distributed ufuncs.

These tests were previously broken.
Some of the tests of bitwise operations had to be removed
because they don't work with the default data.
Add tests for fromndarray and tondarray.
Add previously broken tests.
Change assertions to exceptions.
fromndarray bugfix. Add notes about to/fromarray performance.
Merge pull request #124 from cowlicks/to-from-array

Fix `tondarray` and `fromndarray`