Add last Lua type: userdata. Changed LInstance to inherit from LUserData,

and changed instanceof/casts to LInstance to use LUserData instead. This
should enable various add-ons to be compatible without knowing much, or
anything, about each other.

src/main/java/lua/value/LUserData.java

@@ -0,0 +1,28 @@
+package lua.value;
+
+public class LUserData extends LValue {
+	public static final LString TYPE_NAME = new LString("userdata");
+	
+	public final Object m_instance;
+	
+	public LUserData(Object obj) {
+		m_instance = obj;
+	}
+	
+	public String luaAsString() {
+		return m_instance.toString();
+	}
+	
+	public boolean equals(Object obj) {
+		return (this == obj) ||
+			(obj instanceof LUserData && this.m_instance == ((LUserData) obj).m_instance);
+	}
+	
+	public int hashCode() {
+		return System.identityHashCode( m_instance );
+	}
+
+	public LString luaGetType() {
+		return TYPE_NAME;
+	}
+}