Probe command (fix issue #33)

Here's a first take at adding the probe command. Please review and comment.

How can we use the MockServer to add tests?
According to http://legacy.python.org/dev/peps/pep-0257/ it should be "Build ...", not "Builds ...":

> The docstring is a phrase ending in a period. It prescribes the function or method's effect as a command ("Do this", "Return that"), not as a description; e.g. don't write "Returns the pathname ...".