Make app/credentials.js optional

To match the description on the README

app/config.js

@@ -3,11 +3,9 @@ var coins = require("./coins.js");
 var currentCoin = "BTC";
 
 var credentials = require("./defaultCredentials.js");
-var overwriteCredentials = require("./credentials.js");
-
-for (var key in overwriteCredentials) {
-	credentials[key] = overwriteCredentials[key];
-}
+try {
+  Object.assign(credentials, require("./credentials.js"))
+} catch (err) {}
 
 module.exports = {
 	cookiePassword: "0x000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f",