Add factory for draft and live tags

Modify the existing factory for tags to add a 'draft' and 'live' trait, 
and then use them to define 'draft_tag' and 'live_tag' factories.
Update tests to use the 'live_tag' factory
Set tags to draft by default
Add a state machine to the Tag model

This adds a simple state machine to the Tag model that supports 
publishing a tag from draft to live.

The 'state' attribute has been set as a protected attribute, so that 
it can't be mass-assigned.
Merge pull request #198 from alphagov/add-workflow-to-tags

Add workflow to tags