Multisource support for ViewModel

I hope you will consider accepting this new feature I added to your backbone-view-model.

It adds the ability to derive computed attributes from more than one model. The implementation is backwards compatible with the existing use of source_model merging it with a new `source_models` initialization option.

Passes all original tests unchanged, includes two new tests to cover new functionality.

Readme updated to document new feature.

Cheers,

Andrae Muys