0-10: moving from preview to final in the c++ broker

[Gordon Sim]

I have checked in a change to the amqp.0-10-preview.xml, setting the 
version as 99-0 to distinguish it from the final version recently 
checked in that will remain marked as 0-10.

The python client has a few updates just to allow it to treat 99-0 as a 
'0-10 generation' spec for the purposes of framing etc.

I have also modified the broker to support both 99-0 and 0-10. At 
present the support for each is identical but they are based on two 
separate code paths (Connection/ConnectionHandler/SessionHandler for 
0-10 and a copy & paste of that path, PreviewConnection/ 
PreviewConnectionHandler/PreviewSessionHandler, for 99-0).

The intention is now to start migrating the 0-10 code path to the latest 
version of the spec. The 99-0 codepath will be left alone so that 
clients will be able to continue using the preview functionality until 
they are ready to switch over to the final version (at which point I'll 
delete all the PreviewXXX classes and support for 99-0).