Added fix to use field names rather than field ids for NED votables

This fixes #142.  This is done by using an optional `use_names_over_ids` argument to `astropy.io.votable.tree.Table.to_table()`. Note however that this will work for only versions of Astropy>=0.3 . For astropy versions 0.2.x and lower, this is not supported and so for these versions of Astropy, the older `to_table()` method is called, that still uses the field ids and not the field names as table column names. Also refer to https://github.com/astropy/astropy/pull/609