Added PING, Web Security IG and WebRTC to liaisons list

The charter mentions privacy and security issues. As such, reviews by the
Privacy Interest Group and Web Security Interest Group are strongly
recommended.

I added the WebRTC group as well in the list of groups that develop
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).
Added somewhat realistic milestones for the Presentation API

The milestones take for granted that the Working Group would be
chartered for about 2 years.
Updated mailing-list name to avoid conflict with CG
Added sentence to allow splitting functions into multiple specs
Clarified CC-By and W3C Document License dual-licensing rules

The section is mostly a copy-and-paste of the end of the Document
License section of the HTML WG charter:
http://www.w3.org/2013/09/html-charter.html#documentlicense
Dropped charter authors "TBD" section
Updated URL of group's home page for consistency with mailing-list name
Merge pull request #7 from tidoust/liaisons

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