FIxed GDC compilation (basic betterC WIP) and some improvements

-fixed issue with adding/removing entities inside events handling
-fixed EntityMeta.getComponent() (added check if component_id is valid)
-added function hasComponent to entity to check if component exists

dub.json

@@ -74,6 +74,9 @@
 			"dflags": [
 				"-betterC",
 				"-defaultlib="
+			],
+			"dflags-gdc": [
+				"-fno-druntime"
 			]
 		},
 		{
@@ -88,9 +91,7 @@
 			],
 			"dflags-gdc": [
 				"-fno-druntime",
-				"-fvisibility=hidden"
-			],
-			"lflags-gdc": [
+				"-fvisibility=hidden",
 				"-lpthread"
 			]
 		},
@@ -105,9 +106,7 @@
 				"-betterC"
 			],
 			"dflags-gdc": [
-				"-fno-druntime"
-			],
-			"lflags-gdc": [
+				"-fno-druntime",
 				"-lpthread"
 			]
 		},
@@ -118,6 +117,9 @@
 				"-betterC",
 				"-unittest"
 			],
+			"dflags-gdc": [
+				"-fno-druntime"
+			],
 			"sourcePaths": ["source/","tests/"],
 			"mainSourceFile":"tests/runner.d",
 			"excludedSourceFiles":[