clarification: move extern declarations to headers

+ add 1 space after if, for, while and switch keywords
+ replace some inline functions with macros
improve error handling and debug

-add error tests for memory allocation and SDL functions
-apply "exit on errors" approach instead of succession of returns
-show file and line error when in debug mode (make debug)
-add comments and readability modifications
move uncommon functions away from common.c
organize data files into subdirectories
Merge branch 'master' of github.com:y0uCeF/Ronda

Conflicts:
	player.c