Download fixture data as excel file

[Fogbugz: Download, edit and re-upload fixture data.](http://manage.dimagi.com/default.asp?63330)

- Adds the ability to download all fixture data as an excel-sheet.
- Adds ability to add multiple comma-separated user and group list while uploading through excel-sheet.
    - Currently multiple users or groups can be added only on website through UI. I discussed with Sharon whether such feature is needed. Now multiple users/groups can be added using excel
- Added extra fields (UID, Delete(Y/N)) in excel-sheet for deleting and modifying items.

TO-DO: Currently, only download is supported.  Modifying data using re-uploaded edited excel file is not yet supported. Re-upload of the file deletes and creates entire fixtures, similar to current behavior.