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
Merge branch 'milestones' into liaisons
Merge branch 'license' into liaisons
Updated URL of group's home page for consistency with mailing-list name