Fix for #105: handling of 0-D datetime64 arrays

Wow, that was nasty.

It turns out there appears to be no such thing as a 0-dimensional
np.datetime64 array under numpy 1.8. All my attempts to create one seem to
silently cast it to an integer (although it keeps the datetime64 dtype?!?):
array(1372680000000000000L, dtype='datetime64[ns]')
Add back in DataArray.item()

Fixes #106.