Test proving #191
Tell pexpect how to run execv better. Fixes #191
Changelog re #191
Make Task ok with MethodType tasks

This updates the argspec to allow for the body of the task to be a
method from an object instance, rather than just a function or a
__call__.  For the most part it is pretty straight forward, however the
way the executor._call_objs() test method set things up created a
wierdness in light of accepting methods. Blindly poping the "first"
argument of mock.__call__ pretended self was the ctx arg - not OK.

Rather we needed to create some simple task functions that proxied to a
Mock object to allow for proper semantics.