Started watermark management.
TODO: transparancy, position, scaling
Watermark are now working well and handle transparency on the image
Create "CommandExecuter' class to execute all external programs. Makes the code much cleaner.
Fixed issue with external scripts execution and blocking pipe. Now even without output from external prog, we keep on looping and sending heartbeat !

HouuuuuHaaaaa !
Merge pull request #12 from koxon/master

Watermark are now working well and handle transparency on the image