Clarify the meaning of the second argument in --help output

This adds a command line option -l (--list-payloads) to list all the
standard payloads available in the 'pyrasite.payloads' package.
It also allows the user to refer to those using just the file name without
a directory, e.g.

    pyrasite $pid dump_stacks.py

It's possible to disambiguate standard payload names from local user files
with the same name by using explicit paths, e.g.

    pyrasite $pid ./dump_stacks.py  # runs user script, not standard payload

Fix the context manager unit tests on py2.4

Context manager unit test fixes

Make our unit tests a little happier

Make the PyrasiteIPC a context manager as well.