Squashed upload-api and download-excel into one new branch

Merged two cases corresponding to [Fogbugz: Download, edit and re-upload fixture data.](http://manage.dimagi.com/default.asp?63330) and [Adds 'upload fixtures through API' functionality](http://manage.dimagi.com/default.asp?61884#357275) into one new branch.

Following are PRs for both of them https://github.com/dimagi/commcare-hq/pull/454, https://github.com/dimagi/commcare-hq/pull/439. They can be closed with this new merged PR. Following is the functionalities this PR adds.

- upload excel file through API with following command
  - > curl -F "file-to-upload=@sample_fixture.xlsx" -v --digest -u sreddy@dimagi.com:password http://127.0.0.1:8000/a/{domain}/fixtures/fixap
- download Fixtures as an editable excel sheet, which can be uploaded back after edits.
- edit existing fixture items, by modifying them on excel-sheet.
  - re-upload of new document just creates new item-lists
  - re-upload of edited doc updates existing items
  - multiple users/groups can be added using multiple-columns.

TODO: Editing data-types, adding extra fields to fixtures is not yet supported.