Remove Gemfury.

We don't need it for any of this project's dependencies.
Merge pull request #193 from alphagov/remove-gemfury

Remove Gemfury
Generalise content storage and presentation for specialist docs

All document specific fields will be stored in a Hash field such that all
document types can be handled by a single mongoid class and presenter.

Adding more specialist document types should not require changes to the
Content API.
Merge pull request #192 from alphagov/generalise-specialist-docs

Generalise content storage and presentation for specialist docs