Deprecated 'attributes' in favor of 'attrs'

Also:
1. Don't try to preserve attributes under mathematical operations.
2. Finish up some cleanup related to "equals" and "identical" for testing.
3. Options for how strictly to compare varaibles when merging or concatenating
   (see #25).

Fixes #103 and #104.
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.
Merge pull request #107 from akleeman/fix-attributes

Deprecated 'attributes' in favor of 'attrs'