Live-Better: Write to Data Extension in ExactTarget

An example url would look something like:

https://profile.thegulocal.com/et/subscribe/lb6?returnUrl=http://www.thegulocal.com/lifeandstyle/fitness

Unfortunately this has to be a 'GET' request, as 'POST' requests won't
survive through our login process.

Uses the Play WS http-client for calling Exact Target, rather than Apache.
Merge pull request #3325 from guardian/exact-target-subscribe-data-extension

Live-Better: Write to Data Extension in ExactTarget