COMPAT: add back img._data property for now

The recent refactor removed the ``_data`` attribute, but sure enough,
nipy was depending on it.  Add back ``_data`` but put in a
FutureWarning.  Test.
Merge pull request #193 from matthew-brett/add-warned-data-property

COMPAT: add back img._data property for now

The recent refactor removed the `_data` attribute, but sure enough, nipy
was depending on it. Add back `_data` but put in a FutureWarning. Test.