Allow optional defaults when sending email alerts

gov delivery now allows us to send header, footer, from_address_id and
urgent flags when sending email notifications in order to override defaults
set in their admin interface.
Merge pull request #65 from alphagov/feature/email-alerts-extra-options

Allow optional defaults when sending email alerts
Force application to only respond to JSON

As with all other GOV.UK APIs we should only respond to JSON requests.