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
Update links to documentation

Govdelivery have moved their API docs.
Merge pull request #62 from alphagov/update-docs-links

Update links to documentation