Switch to using superagent so that it's isomorphic

[wheresrhys]

As search is POST based it won't work without CORS, but the GET requests could potentially work on the client side. Would be nice to integrate promises (e.g. https://github.com/wheresrhys/superagent-promises) into it too. Perhaps promisification could be configurable

[richard-still-ft]

I'm not sure this is a genuine requirement. API keys are private and should never be in the browser. There is currently no use case where we would call a core API directly from the browser. @commuterjoy you also raised this as an issue, what are your thoughts?

[commuterjoy]

Yeah, I'd put it low down on a list of priorities until we understand what sort of site we are building. The more client-side rendered code we use the more an isomorphic client library make sense (albeit via a proxy to mask api keys).