Remove reference to 'error' property in documentation

All error responses now use the 'errors' array property, the 'error'
(singular) string isn't used anywhere.

Fixes #337
Add documentation about i18n usage

Some examples and explainations of how to use the i18n functionality of
popit.
Expand description of area object popolo extention

Add some more information about what sort of thing it might be used for,
along with an example of it in action with a MapIt url.
Add documentation for coalitions support

Explain what the coalition support can be used for, how it is used and
provide an example of it in action.
Move Popolo extensions section further down

We don't want to confuse the user with advanced topics before we've
explained the basics.
Merge pull request #377 from mysociety/documentation-updates

Documentation updates
Set default language when calling the API

Eventually this will be configurable through the UI as a per-instance
settings.
Update popit-api
Rename slug_url property to url

Since we're removing slugs from the urls it makes sense to rename this
method in the models.
Use url method in the views
Remove slug handling from models

If a user wants to add a slug field to their models manually, they still
can, but we no longer do any special handling of that field. Also remove
the deduplicate_slug function.
Route to apps using the model id instead of slug

Since the slug is not a unique field in the database and it doesn't
appear in the popolo schema we're replacing it with the id field. Users
can still specify a custom id if they'd like to have "pretty" urls, but
when they do that using the id field the uniqueness is enforced.
Change suggestions list to use id

Sadly we can't use the model's 'url' function in this view because it's
also rendered clientside by Backbone, which doesn't (yet) have that
method on the models.
Update fixtures to use custom ids

This means that we're testing the custom id functionality by default and
also means there's no need to change all the url assertions in the
tests.
Don't ask for a slug when creating records

When creating people or organizations don't ask the user to enter a
slug, instead leave mongo to generate an id for the record. If custom
ids are required then the user has to create records using the API.
Fix grammar in organization template

I've also updated the comment in the template as it took me a moment to
figure out what a 'bb view' was.