Refs #334 Improve delete contact button

In registration contrib app, on the edit contact page:
* Don't display delete button when adding a contact
* Make the "Save Contact" button green so it stands out
* Make the "Delete Contact" button gray so it doesn't stand out
* Make the "Delete Contact" button just popup a dropdown, in red, that
  says "Really Delete Contact" - user has to click a second time on the
  red button before the delete happens.
0.18.0 is next release
Fix links to bootstrap v2.2.2 documentation
Fix link to virtualenv docs
Remove reference to outdated Django Book (Django 1.0)
Fix admin messagelog interface
Comment out PingHandler so tutorial is accurate
Get rid of uses of the deprecated 'msg.connection'
Merge pull request #334 from rapidsms/feature/334-contact-form-delete-button

Contact form: Delete and Save buttons are too similar
Fix links to old Django docs, link to "dev" version instead
Fixed default value for booleanfield, as per django 1.6 booleanfield has a default of none
Merge pull request #431 from jthurner/documentation-small-fixes

Documentation - small fixes
Merge pull request #434 from vkurup/427-comment-ping-handler

Comment out PingHandler so tutorial is accurate
Merge pull request #435 from vkurup/404-connection-deprecated

Get rid of uses of the deprecated 'msg.connection'
Merge pull request #433 from vkurup/432-fix-admin-messagelog-interface

Fix admin messagelog interface
Prepare for Django 1.7 support

As a first step, I want to get tox and Travis in sync.

* Add py27-1.7.X, docs, flake8, and coverage targets to tox
* Remove unnecessary TESTENV variable
* Organize TOXENVs in .travis.yml
* Remove pep8 commands from .travis.yml (now run in tox instead)
* Temporarily allow failures on 1.7
* Add mock as a test_requires
* Clean up the tests/requirements files. Most of this is installed via
  setup.py. Only include optional requirements here (django-celery,
  djtables). Unfortunately, we also have to duplicate the `mock`
  requirement here since tox doesn't read test_requires
  https://bitbucket.org/hpk42/tox/issue/13/tox-should-reuse-tests_require
Fix all flake8 warnings
Improvements based on code review
Merge pull request #445 from rapidsms/feature/prepare-for-17

Prepare for Django 1.7 support
Merge pull request #444 from shanx/develop

Fixed default value for booleanfield, as per django 1.6 booleanfield has a default of none