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
put third-party stylesheets before internal ones
remove (almost) all custom styles
Set default language when calling the API

Eventually this will be configurable through the UI as a per-instance
settings.
add Foundation 5 to the public/sass directory
styling for alert bars and warnings
whitespace cleanup
Fix issue where /about fields default to false

The issue was the all fields were getting cast to String values. When
the page was saved the correct settings were saved in memory, but a
string representation of the settings were saved to the database. So the
problem didn't become apparent until the process was restarted and the
settings were loaded from the database again.
Update popit-api
centre the header, footer, and homepage contents
three column layout for footer content
header and footer refinements
simplified html for homepage when there are people and organizations in the db
better order of summary elements on homepage
whitespace fixes
additional homepage sections for setup and api guides