Update copyright headers to 2013
Add missing copyright headers and some information to LICENSE.txt

Include information about the definition of "MPExtended Developers" in
LICENSE.txt. Also add a CC-zero-like license grant for the Interfaces,
as it's not bound by copyright in most countries anyway, and people
should be free to use the Interfaces DLLs as much as the JSON interface.
Merge branch 'bump-copyright'