Add overrideCallback property to enyo.JsonpRequest and clarify documentation around callbackName use.
Add test for proper overrideCallback usage.

Enyo-DCO-1.1-Signed-Off-By: Ben Combee (ben.combeee@lge.com)
Merge pull request #307 from enyojs/jsonprequest-override-callbackname

Reviewed-By: Cole Davis (cole.davis@lge.com)