Use empty main instead of relying on main generation by compilers

GDC doesn't seem to do this

dub.json

@@ -16,9 +16,7 @@
 			"sourcePaths": [
 				"test"
 			],
-			"dflags-dmd": [
-				"-main"
-			]
+			"mainSourceFile": "test/poodinis/testmain.d"
 		},
 		{
 			"name": "unittestVerbose",
@@ -27,9 +25,7 @@
 			"sourcePaths": [
 				"test"
 			],
-			"dflags-dmd": [
-				"-main"
-			]
+			"mainSourceFile": "test/poodinis/testmain.d"
 		},
 		{
 			"name" : "quickstartExample",

test/poodinis/testmain.d

@@ -0,0 +1,8 @@
+/**
+ * Poodinis Dependency Injection Framework
+ * Copyright 2014-2016 Mike Bierlee
+ * This software is licensed under the terms of the MIT license.
+ * The full terms of the license can be found in the LICENSE file.
+ */
+
+void main() {}