BUG: interpolate.interp2d would ignore three of its arguments

This should solve [#1072](http://projects.scipy.org/scipy/ticket/1072) and [#1444](http://projects.scipy.org/scipy/ticket/1444). I've yet to update the tests, but please tell me if this the right way to go, as I have a few questions remaining.

* I changed the docstring's meaning of `copy=False` to not *promise* a reference to be stored. This matches the old behavior. The reference/copy is useless anyway, as it's never used. Maybe we should get rid of the `x, y, z` members and document `copy` as being a no-op, only accepted for compatibility with `interp1d`?
* The proposed code is backwards-incompatible. Should we add an `extrapolate` argument (or accept `bounds_error="extrapolate"`) to get the old behavior back?
* Finally, should the bounds be public or private (leading `_`) attributes? Does SciPy have any conventions that I should follow here?