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.
Confirmed this was invalid syntax. It needs to be
`scope.eventOpts?.zoomDebounce or DEFAULTS.debounceMs`

original results:

`scope.eventOpts?dragDebounce ? DEFAULTS.debounceMs` yielded

```
if (typeof scope.eventOpts === "function") {
  scope.eventOpts(typeof dragDebounce !== "undefined" && dragDebounce !== null ? dragDebounce : DEFAULTS.debounceMs);
}
```