SetPositioning exposition in the API

The method in ol.Overlay is not exposed whereas is equivalent
getter getPositioning is visible in API and documentation.
The PR add a line of description to the documentation and expose
the method
Accessor for polygon rings
Make geometries event targets

Previously, the tests were using eql to make assertions about matching geometries.  This is inappropriate for structures with circular references (as with goog.events.EventTarget);
Add setCoordinates for point and dispatch change event
Add setCoordinates for linestring and dispatch change event
Change event for polygons
Merge pull request #1094 from ahocevar/master

Fixing tests (see #1093)
Merge pull request #1095 from ahocevar/master

More test fixes (see #1093)
Listen for geometry events and fire feature events
Listen for feature events and fire layer events
Merge pull request #1096 from tschaub/editable-features

Editable geometries and features.
Fire change events for multi-part geometries
Fixing doc comment
We want to create a deep clone, not a shallow one
Merge pull request #1097 from ahocevar/multi-geometry-events

Fire change events in multi-part geometries
Make zoom control animation duration configurable
Make double click zoom animation duration configurable
Make keyboard zoom animation duration configurable
Add support for ol.overlay center positioning
Update overlay example with center positioning