implement alpha-beta-pruning

* as we can now search too a much greater depth in no time
  the strength adaption is removed
move normalization to a file on its own
add more license headers
correct some comments
small ai fix, usually should not matter
spacing and typo correction
add colour
add _POSIX_SOURCE for newer gcc
fix constant buffer length
fix missing pixel in the board
add flushing in the graphic interface to fix hangs
update email address
add ui help
improve move selection for winning moves