Export and externalize Control.setMap
Create ol.inherits alias, and export it
Add a custom controls example

Inspired from @cedricmoullet's ZoomExtent example in https://github.com/cedricmoullet/ol3/commit/3312c37ed0ac93e9b3c2cb0cea76a070ea1c2cf4.
Export Control.getMap
Externalize Control.handleMapPostrender
Use our own null function for handleMapPostrender

If we use ol.control.Control.prototype.handleMapPostrender = goog.nullFunction the API doc doesn't show the mapEvent parameter of the handleMapPostRender function.
Merge pull request #706 from elemoine/user-extensions

Custom control framework