Fix GeoServer format option param key

See http://docs.geoserver.org/latest/en/user/services/wms/vendor.html#format-options
Move serverType option to olx.source.ImageWMSOptions
Export ol.source.wms.ServerType.*
Cast serverType option in examples
Add ol.format.IGC#getExtensions
ol.render.DragBox renamed to ol.render.Box
Use better type
Rename a property
ol.render.Box refactoring
Add ol.interaction.DragBox
Make DragZoom interaction inherit from DragBox
Rename drag zoom interaction exports file
No default style in ol.render.Box
Add style option to DragBox interaction
Add style option to DragZoom interaction
Remove dragbox.exports
Don't attempt to render features will null geometries in ol.render.canvas.Immediate
Don't attempt to render features will null geometries in ol.renderer.vector.renderFeature
Add extra assertion that feature's geometry is not null
Handle feature's geometries changing to and from null in ol.source.Vector