Fixed bug where uninitialized center causes map dragging to fail

I encountered this bug in Chrome 25.0.1364.160 on Ubuntu 12.04 in the simple example demo here: http://tombatossals.github.io/angular-leaflet-directive/

Clicking and dragging the map throws a bunch of TypeErrors because the controller in this example never initialized a map center. Furthermore, after you zoom once you can no longer click and drag the map.

To fix, it now warns if center is undefined and uses $parse(...).assign to safely manipulate $scope.center