Domain layer helpers for Cucumber
Notifications by tag wired up

Yet to test / implement
* Real tag searching
* GovDelivery API support
Clean up Cucumber environment file
Generate our own UUIDs for our objects
GovDeliveryClient can send notifications
GovDelivery doesn't massage api response

It just turns the XML into a struct and makes keys with dashes use underscores
instead, eg `param-id` becomes `param_id`.
Use topic subscription URL from config in service

This link template used to exist in the client but the client has since been
simplified to remove any data massaging.
Implement TopicTagSearcher

+ @evilstreak
Factor out topic tag searching
Merge pull request #11 from alphagov/send-alerts

Send alerts