Add _.pickOwn(object) that returns only own properties

This pull request adds `_.pickOwn` method that returns a copy of the object that excludes properties from the prototype.

I believe there may be many use cases when this would be useful. Here's mine: I'm wrapping JSON objects with classes and treating those objects like smart data. For example a class `Person` may have `getFullName` method. When passing such objects to templates I prefer to strip those additional methods by getting copy of the object without any properties from its prototype.