Updates to liaisons, milestones, document license and mailing-list name

This pull request contains a collection of "small" changes aimed at finalizing the draft charter. While changes are unrelated, it seemed better to group all changes into one pull request for review purpose.

List of changes:

- Added the Privacy Interest Group and Web Security Interest Group to the list of liaisons (I had missed them simply because these groups did not exist last time I prepared a charter...)
- Added the WebRTC group to the list of liaisons as a group that develops specifications that are potentially relevant for implementors of the Presentation API, e.g. for extending the API to support use cases that the group will not address per se (such as content mirroring).
- Milestones section completed, targeting a 2-year-long group
- Added a short sentence that lets the group split API functions into multiple specs if it resolves to do so (in other words to let the group organize its work as it so wishes)
- Clarified the Dual License mechanism in the Document License section, using the same rules as those of the HTML WG
- Dropped the authors section as I was not sure what to put in there. Feel free to yell if you prefer to have something such as "Prepared by the Second Screen Presentation Community Group" or names of individual contributors
- Updated the name of the mailing-list (now public-secondscreen) and the group's home page address not to conflict with the mailing-list that the CG will continue to use