-removed UpdateBySystems version switch (it's now default behaviour)

-beginRegister() and endRegister() was added (every register functions must be called between this calls, update calls can't be called there)
-SystemCallers and Systems dependecies is rebuild always in endRegister();
-beginRegister() clear all SystemCallers and Systems dependencies
-possibility to get System ID and execution state
-JobGroup has now pointer to it's parent (SystemCaller)
-SystemCaller clean up destructor

dub.json

@@ -10,9 +10,6 @@
 	"dflags-posix-ldc": [
 		"-defaultlib=phobos2-ldc,druntime-ldc"
 	],
-	"versions": [
-		"UpdateBySystems"
-	],
 	"configurations" : [
 		{
 			"name" : "library",