Fix Performance issues on drag and zoom

Scope variables like center and zoom are updated on every drag / zoom event, which causes a major slowdown of the map, especially on mobile. Fixed that by changing _.defer to _.debounce with a configurable debounce delay.

See [Issue 734](https://github.com/angular-ui/angular-google-maps/issues/734)

Please test this change, it solves the issues in my case (made the changes in JS), but I wrote the coffeescript without being able to test it, as I could not get the project to build locally.