Add a rails generator for GOV.UK components

Generates all the file scaffolding, in the right places, as
the files are a bit spread throughout static at the moment.

This will include all the files required to let the component
be demo'd on govuk-component

Appending to the json docs file is a bit shonky, but we're
not planning on using that as a docs method long term. It'll
likely be inline with template, or at least one doc file per
component, which will make this simpler.
Make the GOV.UK component doc fixture structure clearer

Fixtures are a hash of hashes, the parent hash is fixture_name => fixture

The fixture is a hash of locals to be passed to the compontent view.
Update readme to include component generator