Added Furl model which creates redirects on save

On save, posts to the publishing api.
relevant docs: https://github.com/alphagov/content-store/blob/master/doc/input_examples/redirect_item.md
Furl manager can accept furl requests
Requester notified when furl request accepted
fURL manager can reject fURL requests (ish)

Does not include setting any kind of state for fURL requests,
so they'll still appear in the pending request index.
Use factories in FurlRequest spec
Add states to furl_request and refactor

Three states - pending, accepted and rejected now exist.
Some logic, such as sending notifications, and setting 
up a fURL is also now handled by the state changer methods.
Furl request index only shows pending requests
Rename Some things

Furl -> Redirect
from_path (FurlRequest and Redirect) -> from_path
same for to_path
Add required 'update_type' field to publishing api hash
Merge pull request #3 from alphagov/activate-furls

fURL manager can accept and reject fURL requests