Strict json response structure

@dschenk / @btemperli I made some structural changes to the error and success responses. To adhere more to the specification here: http://labs.omniti.com/labs/jsend. Especially the error response now has a mandatory ```message``` parameter. From my point of view, we had some inconsistencies in there as for which parameters are always present and which are optional. Was that on purpose that we did not stick to the proposed format from http://labs.omniti.com/labs/jsend? Care to take a look and tell me what you think?

I assume this will break things in ```1.0.x``` so we should probably increase the release number to ```1.1.x``` once ready.