Added IKVM_EXPERIMENTAL_JDK_7 environment variable to enable loading Java 7 class files.

runtime/ClassFile.cs

@@ -85,7 +85,7 @@ namespace IKVM.Internal
 		private static class SupportedVersions
 		{
 			internal static readonly int Minimum = 45;
-			internal static readonly int Maximum = 50;
+			internal static readonly int Maximum = JVM.SafeGetEnvironmentVariable("IKVM_EXPERIMENTAL_JDK_7") != null ? 51 : 50;
 		}
 
 #if STATIC_COMPILER