BUG: add __len__ method to ma.mvoid; closes #576

A record selected from a structured ndarray by indexing is of type numpy.void.  Indexing a record from a structured masked array yields numpy.void if the mask is False, but numpy.ma.mvoid if the mask is True.  This PR adds the mvoid.__len__() method that was missing.  It merely calls the __len__() method on the _data attribute, which is of type numpy.void.

A test is included.