JZ: Rename common.pip to base.pip
JZ: Cap dj under < v1.7 (to get latest 1.6.X)
JZ: Add requirements readme
JZ: Add requirements readme
JZ: Update `.travis.yml` for requirements changes.

requirements changes => #640
Merge remote-tracking branch 'origin/reorganized_requirements'

* origin/reorganized_requirements:
  JZ: Update `.travis.yml` for requirements changes.
  JZ: Add requirements readme
  JZ: Add requirements readme
  JZ: Cap dj under < v1.7 (to get latest 1.6.X)
  JZ: Rename common.pip to base.pip
DW: Added a delete submission endpoint
DW: Added delete submission end point documentation
UK: remove trailing slash
UK: skip _attachments in csv exports, closes #644
UK: ensure we have a unique name when adding multiple select choices as elements, closes #366
UK: _attachment, empty list on csv export
UK: pip install -r base.pip
ME: replaced 'formhub' references with 'ona'