import public domain country flags, ref #156

[illwieckz]

This PR import flags from the Teeworlds project, imported with the tools/import_flags_from_teeworlds.sh script from PR #171.

This flags list is more complete than the previous one, and flags have all the same dimensions (not like the previous ones).

I resized the lan.png flag to fit with the country flags.

I moved the country flags to an additional country subdirectory in pixmaps/flags in source tree with a COPYING file (CC-BY-SA 3.0) inside, so it's easy to know which file is covered by which license.

@jmallach, perhaps these changes will need you to update some legal files in Debian repository since these flags are distributed under CC-BY-SA 3.0. Perhaps the same for @lnussel.

Also, this PR fixes a legal issue: the legal status of previous flags were unknown.

[illwieckz]

OK, flags are basically in public domain, unless some artist add some graphical effect…
I found public domains flags here: http://www.famfamfam.com/lab/icons/flags/ .

They say “These flag icons are available for free use for any purpose with no requirement for attribution”.

Also, as contrary as the TeeWorlds flag pack, this flag pack do not discriminate countries on an opinion basis. We had a country flag that TeeWorlds does not ship and I did not want to have to face a troll if we stop to ship this flag.
They fit exactly our needs, with the good dimensions (no need to transform them), I just transformed the lan.png icon to have the exact same size than them.

[illwieckz]

So I just moved the flags in a pixmaps/flags/country directory, imported the flags and add a COPYING file with the link to download them.

@jmallach, @lnussel, since these flags are in public domain, there is nothing to change in any legal file in any package.