Remove unnecessary sys.path insertion in hydrogen_proxy script.

~/.hydrogen_proxy/modules need not be on the PYTHONPATH when ~/.hydrogen_proxy
is already.
Fix bugs with HTTPRequest's get_status_line() method.

The HTTPRequest's get_status_line() method was apparently untested until I
started writing the unit tests for it.  The "proxy" argument to it wasn't
having the intended effect.
Add unit tests for the HTTPRequest class.