Add Hammer.js support

Hey,

We faced an issue when using Hammer.js and enabling `options.closeByDocument`. The dialog will close immediately if `ngDialog.open` is triggered through a Hammer.js gesture.

I added support for `Hammer(elem).on` and `Hammer(elem).off` if Hammer.js is loaded in order to fix the issue. It will fall back to `$dialog.bind` and `$dialog.unbind` if `Hammer` is undefined.