add PermissionOverwrites class in preparation for permission evaluation

src/structures/PermissionOverwrites.js

@@ -0,0 +1,19 @@
+'use strict';
+
+class PermissionOverwrites {
+	constructor(serverChannel, data) {
+		this.channel = serverChannel;
+		if (data) {
+			this.setup(data);
+		}
+	}
+
+	setup(data) {
+		this.type = data.type;
+		this.id = data.id;
+		this.denyData = data.deny;
+		this.allowData = data.allow;
+	}
+}
+
+module.exports = PermissionOverwrites;