Refactor API before public launch

* moves formatting logic from frontend app to backend
* hides TARIC/CHIEF specific entities and names in the API.
* improves spec coverage
* contains various enhancements and spec fixes

Updates rabl because it added support for partial local variables
Updates therubyracer to avoid gem installation errors, same was done in
static
Expose vat and excise on Measure as booleans
Merge pull request #122 from alphagov/36718669-public-api-refactor

Refactor API before public launch