includes changes for #250, #267. UI change to download in xls format, new management command export_xls, to export twitter-user and twitteruser Set in xls format.Documentation updated to support export_xls availability.
corrected Excel download url to xls, not xlsx
Added csv_headers class attribute to TwitterUserItem
Rewrote export_xls as a wrapper call to export_csv with --xls option
Moved common code for xls and csv generation out of views.py and export_csv.py into utils.py.  Updated export_csv.py to use --xls and --filename parameters. Fixes #271.
Updated export_csv and export_xls to reflect new options added as part of #271
Correction to export_csv file name parameter wording
Made twitter-user option case-insensitive (this was driving me crazy)
Tweaks to export_xls
Merge pull request #273 from gwu-libraries/t271-support-xls-format

 Adds .xls export support.  Fixes #271.