add level completion function

level completion function is implemented:
* upon level loading, amount of destroyable bricks is calculated
* every time a brick is destroyed counter goes down
* once no more bricks are available, game status is changed

so far gameover message is displayed and after 3 seconds
game is reset

closes #4
re-factor several hard coded constants

several hard coded constants, like filenames and numeric values
have been changed to define declarations in header files
e.g. "media/music.ogg" is now MEDIA_MUSIC
add messages for level completion and life lost

new text for level completion and life lost has been added
to media loading routine and destruction routine

a new status of game has been define has GAME_LOST
whenever a player loses a life and game is not over, GAME_LOST
is then the actual status

display of messages according to status have been added to draw
function (level complete and life lost)