Refs #482: new property phantom.casperEngine storing the engine name

It will easier in the code to know the current engine.
Refs #482: fixes utils.betterTypeOf() for SlimerJS

In SlimerJS, the webpage object has no type 'qtruntimeobject' written
in the string form of the object. But it has a __type property.
The function should check this property when the engine is not PhantomJS.