Improvements to the underscore definition.

* Added the OOP version of underscore _(value).
* Pick/Omit accept arrays as well as list of arguments.
* Clone of T now returns T instead of any.
* Removed UnderscoreWrappedObject. It was just a hack to make the test work. Now chain() returns any.
* Test can't test the result of mixin, as new there is the function _() returning the underscore object.