Added _.propertyOf method

_.propertyOf method as discussed in https://github.com/jashkenas/underscore/issues/1916

Resolves #1916
> Only bad ones

That was painful to read... 

> I think it's a code smell to do a test in every call when you could simply do the test once. In fact, it is a code smell whenever you re-compute anything you already know.

I would only consider this a code smell if there was a real perf boost. Since it's not really making a [difference either way](http://jsperf.com/propertyof), I'd argue for it to revert to the original that maintains `length`.