Add a more general purpose "merge" method. 

If we have a "merge" method, it fully duplicates the functionality presented in https://github.com/Wolfy87/Heir/pull/4, but in a much nicer way. 

I also added `heir.hasOwn`, as a shortcut for `Object.prototype.hasOwnProperty`. The "why"s for that are explained in the method's DocBlock.
Fix a little bit of layout
Styling tweaks for the merge pull request.