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 by having extra column for each user/group. There can be any extra number of columns needed. Now a single column with comma-separated user/group list can be uploaded.
- 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. 
- Needs UI tweaks and help-text on how to use excel edit-upload feature

[A sample excel-sheet that is downloaded this way, which can be uploaded back](https://docs.google.com/a/dimagi.com/spreadsheet/ccc?key=0Amg6qlUhNSXndFQ4WGtxZnZJQVFvbm92TWxTQ1ozSUE&hl=en#gid=1)