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.
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