Prepare fluent API for REST client

[romanstrobl]

Currently there are many methods available for slightly different versions of HTTP method calls.

Let's prepare a fluent API, which will allow calling the client like this:
get(path).queryParam(q).headers(h).execute()
get(path).nonBlocking().execute(onSuccess, onFailure)
post(path).body(request).responseType(type).execute()

[romanstrobl]

The current API is good enough for now. We can add the fluent / request builder interface in a future release.