Some new functionality

[harumando]

Hello.
So i've added 2 new functions to this controll:

1. Control can now be displayed as a reversed control. With passing the reversed:true option you can display control upside down.
2. Something called "zoom presets". What it does is it adds new interface elements (buttons) to zoom control. You can click on a zoom preset button and it'll zoom you to a corresponding zoom. The example is given in the "examples" folder. You can initialize zoom presets by passing "zoomPresets" option to the control. "zoomPresets" action is an array of objects:

zoomPresets: [
  {label: 'World', zoom: 0},
  {label: 'Country', zoom: 5},
  {label: 'City', zoom: 12},
  {label: 'Street', zoom: 16}
]

I also added a transition property to the knob so it'll move more smoothly.