Mutable symbolizer properties for style functions

This change adds setters for symbolizer properties. In addition, it
introduces a mutable flag on all styles. By default, this is set to
true. ol.style.createStyleFunction sets it to false for all static
styles.

The new setters assert that the mutable flag is true, so whenever an
application tries to set a symbolizer property on a style that was
assigned to a vector layer or feature overlay, the assertion will fail.
Additional documentation
No need to have a mutable_ flag

Instead, educate users to call setStyle.
Doc improvements
Rename ol.Observable#dispatchChangeEvent() to #changed()

Since dispatchChangeEvent does not only dispatch a change event, but
also increases the revision counter, the method name should reflect
this.
Update docs for changes proposed in #2684
Use ol.LEGACY_IE_SUPPORT and ol.IS_LEGACY_IE define

See #1605
Unset z-index in overlay example
Merge pull request #2692 from fredj/overlay-index

Unset z-index in overlay example
Change signature of ol.layer.Group#getLayers

This commit changes the signature of ol.layer.Group#getLayers from {ol.Collection.<ol.layer.Base>|undefined} to {!ol.Collection.<ol.layer.Base>}. In this way the caller is guaranteed that getLayers returns a dereferencable object.
Remove undefined from ol.Map#getLayers return type

It's already assumed that the collection is defined in ol.Map#addLayer
and ol.Map#removeLayer.
Merge pull request #2675 from fredj/map.getlayers

Remove undefined from ol.Map#getLayers return type
Merge pull request #2687 from fredj/legacy-ie

Use ol.LEGACY_IE_SUPPORT and ol.IS_LEGACY_IE define
StaticVector is not an abstract base class
Add vector layer in export-map example
Merge pull request #2702 from fredj/export-map

Add vector layer in export-map example
Merge pull request #2698 from ahocevar/feature-not-abstract

StaticVector is not an abstract base class
Use pool thread for calling bin/check-example.js.

Saves 50s out of 60s on my machine, that is 83%.
Fixes https://github.com/openlayers/ol3/issues/2695
Merge pull request #2704 from gberaudo/pooled_example_checks

Use pool thread for calling bin/check-example.js.
Auto configure ol.format.GML if not configured with a featureNS/featureType