Write API for Trade Tariff Admin app

This change is for https://www.pivotaltracker.com/story/show/47984981 and https://www.pivotaltracker.com/story/show/52900709 and is related to https://github.com/alphagov/trade-tariff-admin/pull/1.

See the other pull request for more info.

Environment variables that need to be set for this to work:

```ruby
TRADE_TARIFF_USER=  '' # user for HTTP basic auth to the backend
TRADE_TARIFF_PASSWORD= '' # password  for HTTP basic auth to the backend
TRADE_TARIFF_OAUTH_ID= '' # has to be generated using signonotron
TRADE_TARIFF_OAUTH_SECRET= '' # has to be generated using signonotron
```