Updated docs to put e-mail in flow, implemented e-mail testing via admin
Merge pull request #239 from theguardian/email-updates

Add "test e-mail" interface.
Add redis as a requirement; use a Celery transport.
Generate statistics in a background task.

Also updates other tasks to use the shared app
decorator (so that settings are correctly propagated
to unittests).

Also adds a custom test runner to run celery tasks
synchronously.

Issue #214.
Show a warning in case of redis connection problems.
Suppress errors that occur when scheduling tasks.

None of these background tasks are essential to normal
operation, so better to suppress these errors rather than
blow up the request.

Implemented centrally as context manager so that we can tweak the
exact policy later (different celery brokers would throw different
exceptions).

Errors are shown in the cache log handler.