Pass base urls to the API app

This gives the API a simple way of knowing where the main application
is hosted and where the API is mounted within it. This is
used to generate links in the API output.
Update popit-api to latest master

This version of popit-api correctly outputs memberships when the
querying the API.
deploy to production - version bump to 0.0.20
Merge pull request #301 from mysociety/api-base-url

Pass base urls to the API app
Update popit-api
Update popit-api to incorporate api-links fix
deploy to production - version bump to 0.0.21
Ensure base_url and api_base_url is always set

It's not entirely clear to me why this didn't work the way it was, but
it seemed to cause issues with the root route of the API app. By adding
the middleware to the app that mounts the API we workaround that issue.
Update popit-api dependency
deploy to production - version bump to 0.0.22
Hotfix for extra slashes in API links
rebuild npm-shrinkwrap.json
deploy to production - version bump to 0.0.23
Pin gem versions

This prevents errors occurring with the travis build when new versions
of gems are released.

Fixes #331
Fix contributions link in API bindings docs

Closes #286
Fix link to dev blog in contributing docs

Closes #287
Replace run-on sentence with two seperate ones

As suggested in 4e0fdf8.
Change examples to za-peoples-assembly

Replace kenyan-politicians examples with za-peoples-assembly ones.

I also removed the note on the URL potentially 404ing if the data has reloaded, as that seemed more of an issue for auto-generated IDs than pre-constructed ones.
The API now returns membership information

Update the example JSON responses to show the membership information that is now included.
Remove quoting of slashes

Some links had needless \/s