fix($httpBackend): cancelled JSONP requests will not print error in the console

When you cancel a JSONP request, angular deletes the callback for it. However the script still executes, and since the callback is now deleted and undefined, the script throws an exception visible in the console. The quick fix for this is not to delete the callback, but replace it with `angular.noop`.

Closes #5615
Closes #5616
docs(script): add more detailed information

Closes #5671
Closes #5676
Merge branch 'master' of https://github.com/angular/angular.js
componentizd!