Automatically instantiate collection if passed as function reference.

Hi,

I don't know if this is a good idea, or the right way to do it, so feel free to reject this PR if so.

I was a bit confused at first when trying to use Backbone.VirtualCollection because I was passing the function reference of the collection in rather than an instantiated object. I've added a condition to check if the collection is a function and if so, instantiate it.

I've also fixed a few spelling mistakes in the readme file, if that's okay.

Thanks!
Matt