Tests are not actually ran on appveyor for now, since they all fault for
some reason. For now, we just build them. Also, some tests are disabled
on msvc entirely, because they don't even compile. We'll need to look
into those, later. They are disabled using `MSVC_DONT_BUILD`.
Also added a `tox_options_copy` function for cloning an options object.
This can be useful when creating several Tox instances with slightly
varying options.
Previously, all log messages generated by tox_new (which is quite a lot)
were dropped, because client code had no chance to register a logging
callback, yet. This change allows setting the log callback from the
beginning and removes the ability to unset it.
Since the log callback is forever special, since it can't be stateless,
we don't necessarily need to treat it uniformly (with `event`).
