Minor apidoc updates
Merge pull request #1378 from fredj/apidoc

Minor apidoc updates
Make ol.Object#accessors_ a private renameable property
Make ol.Object#listeners_ a private renameable property
Use goog.object.get to suppress compiler warning about use of [] with struct
Make ol.Observable a @struct
Merge pull request #1379 from twpayne/observable-struct

Make ol.Observable and ol.Object @structs
Merge remote-tracking branch 'openlayers/master' into vector-api