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

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)
```