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