Use @classdesc notation

Phew! When I offered to add `@classdesc` to classes https://groups.google.com/forum/#!topic/ol3-dev/7l9rQ6W5c7A I hadn't realised there were quite so many. 113 files updated, some using existing description, some new. Some of these are really stating the obvious, but can be changed later as needed.

Some of the vector base sources I have left for now, while I work out how they fit together. And some others I have left, as I don't know what they are for or when you would use them. These are:
* ol.ImageTile
* ol.Kinetic
* ol.render.canvas.Immediate
* ol.Tile
* ol.TileCoord
* ol.webgl.Context

I added the word 'abstract' to those I consider to be abstract base classes, not usually instantiated in apps. jsdoc does process the `@abstract` tag, but there are issues with this at the moment. In any case, I think these should be differentiated in the html somehow. I'll raise a topic on the list for this.

There's a generated output at http://probins.github.io/classdesc/apidoc/