Allow bound methods as arg for single method interface param.

Java widely uses single method interfaces (such as Callable or
Runnable) as parameters for methods, such as setting up a
callback. Jython since 2.5 has supported passing Python functions as
such arguments, which avoids having to define a class extending the
single method interface. This change allows bound methods to be used
in a similar fashion, by using PyMethod#__tojava__.

Note that any callable object should have this behavior, since there
is no possibility of ambiguity; however, this change did not add such
support for __call__ since it seems to be a bit more complex in how it
interacts with class derivation. But a test, currently skipped, as
been added for such future support.

Partial fix of http://bugs.jython.org/issue2115