Add marker anchor support

[milare]

Implements #26

[johnpryan]

Thanks for the contribution!

Just a heads up, we are going to also make the offset calculation automatic:

      new Marker(
          width: 80.0,
          height: 80.0,
          point: new LatLng(53.3498, -6.2603),
          builder: (ctx) => new Container(
                child: new FlutterLogo(
                  colors: Colors.green,
                ),
              ),
          anchor: MarkerAnchor.center),

so that users won't need to think about dividing 80.0 / 2 and setting the anchor point manually.

Let me know if that doesn't work for your use case or you have any questions!

[johnpryan]

New API: 48a5a3e

[milare]

That's even better, thanks!

[Leffe108]

It may help other people to know that this is how you specify anchorPos to Marker constructor.

Marker(
  anchorPos: AnchorPos.align(AnchorAlign.bottom),
  ...