Add a useful agent.UserAgent.__repr__()

[goodboy]

For debugging purposes, something along the lines of the following will be useful:

>>> sipp_scenario.agents    # lets see the effective arguments we're use to build commands from:
... [u'uac': Agent(timeout=1000,
                   global_vars={u'foobar': u'Hello World',
                                u'baz': u'All hail pysipp'},
                   proxy=('vortex.qa.sangoma.local', 5060))
     u'uac': Agent(...)]

Thanks to @vodik for the suggestion.