revent: No more Event __init__ for subclasses

If subclasses of Event wanted to override the constructor, they used to
be required to call the superclass constructor.  No more!
Remove calls to Event superclass constructor

This is no longer required.
messenger: Make test_client a non-component

test_client.py isn't a POX component, but someone might think it is.  We
now make very clear that it isn't.
packet.icmp: Add Time Exceeded message

This also does some minor cleanup.

This is part of an old and forgotten set of patches which had never been
committed and may be incomplete or have problems.  Please submit bugs!
openflow.nicira: Update some comments
packet.tcp: Refactor option parsing

* Unknown options are now preserved
* Unpacking now structured a bit more modularly (and like libopenflow)

This is part of an old and forgotten set of patches which had never been
committed and may be incomplete or have problems.  Please submit bugs!
packet.tcp: Add MPTCP options

This is part of an old and forgotten set of patches which had never been
committed and may be incomplete or have problems.  Please submit bugs!
openflow.nicira: Add NXM_NX_TCP_FLAGS

This is totally untested.  The basic support should work.  Whether
the mask-checking works is totally up in the air.