v0.2.1_update_2017

This update should support tax year 2017 schemas 2.0, 2.1, 2.2, 2.3. At present settings.py includes a listing of each legal version, but we're likely to switch to a listing of legal years.
see more about the metadata changes here jsfenfen/990-xml-metadata@20255e4
and here jsfenfen/990-xml-metadata@d2f58d6