Restrics triggering of alerts to messages tagged to topics

Alerts should not be triggered for messages regarding major changes
to documents that are not tagged to topics.
Move checking of topics into MessageProcessor
Ensure checking for topics or tags key in document doesn't raise errors
DocumentValidor#valid? returns false instead of raising an Exception
Simplify DocumentValidor#valid?
Merge pull request #14 from alphagov/restrict-triggering-alerts-to-messages-with-topics

Restricts triggering of alerts to messages tagged to topics

https://www.pivotaltracker.com/story/show/79009772
Ensure errors include document representation

Errors now include either the document json paylod from Rabbitmq message
or the parsed json from the system.
Merge pull request #15 from alphagov/ensure-errors-include-documents

Ensure errors include document representation
Strip blank tag arrays.