Added _.propertyOf method

_.propertyOf method as discussed in https://github.com/jashkenas/underscore/issues/1916

Resolves #1916
> Only bad ones 

It still removes them by default. By the way that option was [recently documented](https://github.com/mishoo/UglifyJS2/commit/a64bdda9ae42ff39543fcfe16d70c4e064780e92) too.