Added _.serialize(): Serialize key-value pairs of an object into urlencoded format

Extended Objects with a serialize method, which has long been missing imo. Tend to write this into a utility function or a few lines of code, where it's not appropriate to use $.param(). Would prefer to see it readily available in this library, although it's more abstracted than some other Underscore methods. Otherwise, it could be demoted and qualify as a useful _.mixin that I'll commit elsewhere.