added a "world thread" which automatically stops when World.clean() has been called

This thread does nothing yet, except sending an INFO level debug message every second. The goal is to prevent the world simulation from slowing the UI.
now commands are stored in the world queue before being executed

Note: the game might have a strange behavior because the events are still rendered from the world thread!
removed safe_update()
bug fixed: impossible to save a game

Now the command queue is excluded from the pickle. Thanks to Christopher Toth for reporting the bug.
now "direct server events" are stored in the client queue before being rendered
Fixed build process to work with Python 2.7

(thanks to Christopher Toth for the patch)
updated Slovak and Czech translations

thanks to Marco Oros
added "Making AIs Tutorial"

thanks to Thiago Seus
if the game is starting, the "no unit defined" warning won't be issued
Merge branch 'fast-ui'
bug fixed: the full screen mode wouldn't work if the video card maximum resolution is too big for the monitor
updated README.txt and removed some unused or obsolete scripts
avoided the warning about missing "proportion_-1" value in style.txt
now ConnectionToServer.read_line() returns None or a string

(instead of returning an empty list or a list containing one string)
This should make the code clearer.
updated Brazilian Portuguese translation

thanks to Thiago Seus
added race menu for guest player
fixed synchronization warning (the fast UI caused a false positive)
make the difference between a temporary time mismatch and a more serious synchronization error
bug fixed: impossible to restore a game

The world thread wasn't restored. Thanks to Thiago Seus for reporting the bug.
moved all server events to the queue