Handle the full layer attribution information

With this PR:
 - the OpenLayers.Layer attribution property accepts a string or an object. This object is the same as one gets when parsing WMS Capabilities: ```{title: "my compagny", href: "http://my.compagny.com", logo:{format:"image/png", width:10, height:10, href:"http://my.compagny.com/logo.png"}}```,
 - the OpenLayers.Control.Attribution has been overhauled to accept both attribution types. In the case of an object, the attribution string becomes an attribution link, which can be customized by the way of the ```layerTemplate``` api property,
 - the OpenLayers.Format.WMC is able to serialize / deserialize the full attribution object. This information is stored in the Layer extension, since it is not standard.

Tests pass, please review.