only initWrapper() once.  make wrapper constructor accessable as `_._wrapper`.

Previously, there was no way to get at the OO wrapper object.
So if a user added a custom method on to _, or defined a custom
alias, it would not be reflected in the wrapper.  Now it is at
least possible, though there is no API to do it.  I alluded to
making one in my commit that added _.alias() (which I recently reverted,
as it added weight to the stripped version), but honestly I think
the wrapping behavior should go in to a seperate library, not because
it is bad, but because it is an awesome pattern that I want to use
on objects other than underscore.
(cherry picked from commit b8fd129130f0956b43d5336452b1a5f709685b55)