Use constant for CSS class 'ol-unselectable'.
Merge pull request #352 from marcjansen/unselectable-css-const

Use constant for CSS class 'ol-unselectable'.
Always render map when layers are added or removed
Merge pull request #297 from twpayne/fix-297

Canvas map is not immediately updated when layers are added or removed
Merge branch 'master' of github.com:openlayers/ol3 into vector