Support TOR_PT_PROXY when acting as a managed client.

This requires pyptlib >= 0.0.6 and obfsproxy >= 0.2.9, along with the
new dependencies (txsocksx, Twisted >= 13.2.0).  The first two are
absolute (an exception will be raised if not met), the latter two will
soft fail (Tor configured with fte + outgoing proxy will refuse to
attempt to bootstrap).