Set up Signon to work in preview and production.

- Configurable URLs
- Configurable tokens

@alexmuller, @maxfliri
Missed a failing test because of Signonotron2 signature
Gitignore dev config (tokens and such) to stop overwriting them.

Sorry I've broken your development environment.
To get it working again just rename the sample file.
Run Travis with the correct (?) environment variable

@robyoung, @alexmuller
Try/catch ImportError for a great dev environment

@robyoung has come up with what looks like a winning
way to manage the dev environment.
Set GOVUK_ENV to test when running unit tests

@robyoung, @alexmuller
Update README
Ensure all admin interface is under one path.
Use backdrop admin ui hostname

Use backdrop admin ui hostname for signon and csv uploads

@alexmuller
@pbadenski
Merge branch 'master' into signonotron2-preview-production

Conflicts:
	backdrop/write/config/development_tokens.py
	backdrop/write/config/test.py
	backdrop/write/config/test_environment.py
	backdrop/write/sign_on.py
Rename sign_on to admin_ui

- Ensure that everything relating to the admin UI is in one file
- This should (?) make it easier to extract to a separate project in future
Merge pull request #98 from alphagov/signonotron2-preview-production

Set up Signon to work in preview and production.