Hi,

I've added a new function called setup_master. 

This function is responsible for returning an usable master to the
caller, or fail and say so. The function is useful for any wrapper
basically, since all them has to set up a master sometime (or maybe
always require a master, but that is not very nice) and will usually do
it half wrong. The half wrong is about relying on _default_root being
available in Tkinter, which is not the case if support for default root
has been disabled.

There are two patches, the first adds the function, the second applies
it self where necessary in Tkinter. The later also adds some new
behaviour in Tkinter, previously Variable class and subclasses wouldn't
work properly without prior creation of a master (there are similar
problems in other parts too).