Introduce 'Mapable' mixin

[chrismayer]

This adds a mixin Mapable, which can be used to bind the OpenLayers map instance to any component.

To use the mixin just import it and add it to the target component, like this:

import { Mapable } from '../../mixins/Mapable';

export default {
  directives: {...},
  mixins: [Mapable],
  props: ...
}

The mixin also checks if the target (father) component has method onMapBound and executes it after the OpenLayers map instance has been bound.

Closes #10