Use ol.extent.* functions in ol.source.MapGuide
Add displayDpi option to ol.source.MapGuide constructor
Add ol.source.MapGuide.getScale function
Merge pull request #1388 from fredj/mapguide-cleanup

Mapguide cleanup
Add ol.structs.RBush#getExtent
Merge pull request #1391 from twpayne/rbush-get-extent

Add ol.structs.RBush#getExtent