nmsg_msgtype.pyx: added types member with list of all available message types

[hstern]

This is not functionally necessary but it will make nmsg easier to use. I've added a dictionary of vendors and msg types. A user can see which message types are available with code like this:

import nmsg
for vname,mnames in nmsg.msgtype.types.items():
    for mname in mnames:
        print '%s.%s' % (vname, mname)

[edmonds]

hm, i'd prefer to put this outside of the nmsg.msgtypes.* namespace, actually. but in general i like the idea of being able to get at the vendor / message type ID mappings somehow.