Refs #312: Build docs cleanly

* Fix all the warnings, even in nitpick mode:

** Give each reference a unique name
** Add intersphinx config so we can link to Django settings
** Add blank lines in nested list (RsT insists on them)
** Remove some links whose targets no longer exist

* Change Makefile and tox.ini to always build docs in nitpick mode
  and treat warnings as fatal
* Change .travis.yml to use tox to run tests, but in a way that still
  lets Travis distribute the work across multiple systems if it wants

Branch: build-docs-cleanly
Test each tox env separately to reduce needed disk space
Branch: build-docs-cleanly
Need pep8 installed for the after_script
Branch: build-docs-cleanly
Merge pull request #312 from rapidsms/build-docs-cleanly

Docs should build clean