Now ssh requires a tty allocation. This presents a scalability problem,
a future version could selectively allocate a tty only if typing
passwords is desired.

Sudo's tty handling is now moved into mitogen.master.

* Support passing Context() objects in function calls and return values.
  Now the fakessh demo from the documentation index would work
  correctly.

* Since slaves can communicate with each other now, they should also use
  the same approach to unpickling as the master already used. Collapse
  away all the unpickle extension crap and hard-wire just the 3 types
  that support unpickling.

This should be pretty much identical the same behaviour as before, but
the extra assertion makes me feel happier.