enabled fixed callback for jsonp calls for layergroups

the task is simple: 

use fixed callback names for jsonp request for layergroups

the problem is if there are more than one layergroup per page, in this case we calculate the callback name depending on the crc of layergroup so it's unique. Also we increment a number so same layergroup requests don't confuse their callback

For some reason jquery does not manage that situation well, neither reqwest so that's the solution for the moment.

reqwest needed some update: https://github.com/CartoDB/reqwest/commit/5492af350d2b0f82cd6efab7cf619f6506f97bd4
Another suggestion ... have a `repr` method so you pass that into uniqueCallbackName and hides what is the representation for the model.