UK: convert date and datetime fields to string for SAV exports, specifically today fields or fields with date type
UK: use six.string_types, for python2/3 compatibility
UK: move encode_if_str out of class methods to make it reusable
UK: encode date strings
UK: encoding of date objects to be optional, csv seem to handle date objects, sav does not
UK: added lines, pretext to rerun travis test
UK: use url name to generate the download json form link
Merge pull request #250 from onaio/sav-date-object-fixes

Sav date object fixes