fixes #1165

Uses builtin AUTHORS THANKS and COPYING files on every platform
Don't ship AUTHORS.txt and THANKS.txt on windows packages
COPYING.txt is kept since it's used in the nsis installer
Only show authors' name instead of the full AUTHORS file
Removes an empty line in THANKS to make it easier to parse without using endless pipes

Now it should not be possible to guess other people's ID.

so that we can update individual Makefile.am files.