Add API request forwarder using HTTParty

This change is for https://www.pivotaltracker.com/story/show/36718669

Plain Net::HTTP has weird issues in resolving tariff-api.dev.gov.uk in local VM. It works with localhost:3018 (tariff-api host), it does not work with full host name even though it is present in /etc/hosts. Just timeouts.

Works fine with HTTParty though and we have it as dependency already.

Currently only allowing GET and HEAD request, read-only.