Add API request forwarder using HTTParty

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
Merge pull request #91 from alphagov/36718669-public-api-forwarder

Add API request forwarder using HTTParty