screencapture in rest service

hi!

I have made a rudimentary REST service to take a screenshot of a running galicaster. I submit it now to prevent us from doing double work. I'll work more on it after easter, as well as adding diskspace and cpu load services.

https://github.com/uib/Galicaster/commit/c616263a0cd7766756800bfac598777ccc3757e4