mod_multicast: completed XEP33 support

During the past week I fixed several small problems on the source code of mod_multicast:

• Periodically remove from the cache those items that are really old.
• If the packet has wrong xmlns, no 'addresses' or no 'address', it now rejects to process the packet and reports an error to the sender.
  
• Fixed reception of IQ Query packets. This topic deserves a blog post, even a documentation section.
  
• Added support for XEP-0092: Software Version. Currently it shows the SVN revision.
  
• I verified that mod_multicast does not interfere with Privacy Lists. I initially thought that problem could appear when sending a multicast packet internally on ejabberd. Actually, everything seems to work correctly.
  

So now I consider the implementation of mod_multicast to be completed. At least it supports XEP-33 v1.1.

The next steps are: active server multicast, and abuse/spam prevention.