Fix getImageElement calls in canvas and webgl renderer
Merge pull request #1184 from oterral/get_image_element

Fix getImageElement calls in canvas and webgl renderer
Add jsdoc plugin that maps @todo tags to @stability and @observable tags.

This is a temporary measure until Plovr handles custom tags or is replaced in the build system.

Add support for indicating API stability with inline documentation

In preparation for documenting the stability of API features, this commit adds support for indicating API stability via a new jsdoc tag @stability.  There is a small plugin for jsdoc that adds a new tag, @stabiility, with some ability to enforce specific level names.  The templates and css have been updated to include the stability level in the generated documentation. (+1 squashed commit)
Squashed commits:
[d6bc03d] Add jsdoc plugin for documenting observable properties of a class.
Merge pull request #1180 from pagameba/doc-jsdoc-plugins

Add jsdoc plugins for stability and observables.
Add `@todo observable` documentation for observable properties

This PR adds documentation for observable properties, which will then be pulled into the docs correctly once #1180 is merged.  This is a first pass based on searching for definition of observable properties being defined as enums after lines ending with `Property = {`.  If there are observable properties implemented that don't follow this pattern then they are not included.

I've added simple descriptions based on what I know or could easily figure out, there may be some properties (like preload) that are not correctly described.

I've also added `readonly` annotations where I knew that a property was readonly.  I may have missed some readonly properties.

ol.layer.Base has a bunch of properties but I don't think it is exported so the documentation of these properties will not show up, so I added the documentation to ol.layer.Layer instead even though this isn't really where it should be documented.
Merge branch 'doc-observables' of github.com:pagameba/ol3 into doc-observables