Create topic feature

* Create topic end-to-end
* GovDeliveryClient
* Needs to handle topics that already exist
Parse the GovDelivery API response to get topic id
Rake file to run specs and cukes by default
Lock down Faraday version
Refactor configuration and add startup.sh
Refactor topic subscription link generation
Respond with 201 on creation of topic
Define unique tag set behaviour
Persistence aspect
Ensure unique topics have unique tag sets
Clear the persistence between tests
Improve mock GovDelivery object id generation

Generate IDs as a deterministic series rather than hardcoding of a single
value which actually renders tests useless.
Merge pull request #1 from alphagov/topic-creation-endpoint

Topic creation endpoint