make _.initWrapper() that builds the wrapper prototye

The intention is that underscore Consumers MUST call _.initWrapper()
in their code to allow the OO style usage, or a runtime error is thrown.
This would break backwards compatability, so for the moment I call
initWrapper myself at the end of underscore.

However, if I comment out my call to initWrapper and don't call it
in user code, then the compiler can strip an empty underscore with
no user code from 2137 -> 100 bytes.  Making the user explicitly say
they want OO wrapping is the only way to achieve this, otherwise the
compiler will always keep the OO wrapper code around.
(cherry picked from commit f591f685466bc04d52b932b021e8d72b7225a6ea)