Add jsdoc plugin for documenting observable properties of a class.

This adds a jsdoc plugin for documenting class properties annotated with `@observable`, following similar rules to regular properties for type and name annotations.  For instance, ol.View2D might get something like this:

    * @observable {ol.Coordinate} center the center of the view
    * @observable {ol.proj.Projection} projection the projection of the view
    * @observable {number} resolution the resolution of the view
    * @observable {number} rotation the rotation of the view in radians

In addition, if the description starts with the word `readonly` then the property is documented as not having a setter.  The output contains a section at the same level as members and methods (and before them) with a properties table of these observable properties.  The table, as proposed, contains Name, Type, Get (always yes), Set (no for readonly, yes otherwise), Event (change:<name>) and Description (with readonly removed if it was present).

I will follow up with two other pull requests, one for actually adding the documentation as `@todo observable ...` and a second which will contain another plugin to map `@todo observable` and @todo stability` to the appropriate things as suggested by @tschaub in #1172.