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