object dtype

Stephan Hoyer on the ML:

> This would be marginally useful -- eventually someone is going to want to
> store some strings in a sparse array, and NumPy doesn't handle this very
> well. Thus pandas, h5py and xarray all end up using dtype=object for
> variable length strings. (pandas/xarray even take the monstrous approach of
> using np.nan as a sentinel missing value.)
