Reset MockDeliveryClient properly between tests

Grouping the ivars should make it harder to do wrong in future
Remove empty created_at field from persisted data

There is currently no implementation to populate it.
Search subscriber list by tag basic implementation

* This will probably break if tags values are given in the wrong order
* Does not handle case if not found
Extract Sinatra adapter for better interface segregation

Sinatra implements many methods and you sometimes find yourself tripping over
them when they all exposed to the application services.
Tweak this hash comparison enhancement as it can be confusing
Implement not found behaviour for subscriber list search
Tag normalization for subscriber list search
Ensure duplicate lists cannot be created

using differently ordered tag values
Add some notes about better revealing intentions
Merge pull request #13 from alphagov/get-subscriber-list

Get subscriber list