The "itemView" property can now take a function.

*NB*: Tests need to be written for this yet.

Normally the `itemView` property of a `Thorax.CollectionView` expects either a string or a `Thorax.View` constructor. We just tweak the checks in `getViewInstance` to be smarter about detecting whether the function is a constructor of a `Thorax.View` or not; if it is a constructor it behaves just as before, and if not we attempt to invoke it as a method of the parent view (by passing `this` through).