More January Sponsored Twisted Development

First, a list of tickets I worked on:

• #2036 - trial runs tests from .pyc files even if there is no .py file
  
• #3575 - Common implementation of RFC 2617 digest authentication (closed)
  
• #3582 - Improve SIP URI parsing/formatting
  
• #3584 - SIP transport and transaction layer
  

It is the last of these on which I spent the bulk of the time, implementing the lower layers of SIP message handling as described in RFC 3261. As far as I know, this is the first implementation of this protocol in Python that has thorough unit test coverage! While reviewing the code from Sine, Divmod's SIP implementation, and comparing to the spec I discovered some significant misunderstandings that I had the chance to fix here. This code lays the foundation for both user agent code (like VoIP clients or SIMPLE instant messaging) and proxies.

Thanks to the SFC and all of the sponsors who made this possible, as well as to all the other Twisted developers who helped out by writing or reviewing code.