move Zendesk ticket raising to background

before this change, tickets were raised in Zendesk synchronously.
This change:

* introduces 'sidekiq' (http://sidekiq.org/), a redis-backed
background worker queue
* moves raising the tickets to the background

The motivations for this change are:

* to respond snappier when the user submits a form
* to not lose user submissions when Zendesk is down
remove javascript associated with zendesk_error page
add endpoint for checking queue lengths
ZD exception test: functional => unit
sidekiq now uses existing redis.yml configuration
log queue lengths with statsd when new ticket raised
clarify copy on acknowledgement
adding missing newlines
update README with background processing task
Merge pull request #76 from alphagov/background-raising-of-tickets

move Zendesk ticket raising to background