Add options object to ol.render.FeaturesOverlay constructor
Export ol.render.FeaturesOverlay
Merge pull request #1433 from twpayne/vector-api-export-featuresoverlay

[vector-api] Export ol.render.FeaturesOverlay
Fix StyleFunction type namespace
Add ol.render.REPLAY_ORDER
Replay groups in defined order
Add ol.FeaturesOverlay#addFeature
Add ol.FeaturesOverlay#removeFeature
ol.FeaturesOverlay creates a features collection

ol.FeaturesOverlay creates an empty collection of features if the features option is not specified.
Merge pull request #1479 from twpayne/vector-api-draw-order

[vector-api] Draw vector features in the order polygons, lines, images
Use FeaturesOverlay in vector-layer example

This demonstrates how to use ol.FeaturesOverlay for feature highlighting.
Merge pull request #1480 from elemoine/vector-api-features-overlay

[vector-api] Add ol.FeaturesOverlay#addFeature and #removeFeature
Add ol.BrowserFeature.BACKING_STORE_PIXEL_RATIO constant