Only require the API/Express when using it

Fixes an issue with the Tessel, which cannot load Express.

lib/cylon.js

@@ -10,8 +10,7 @@
 
 var Async = require('async');
 
-var API = require('./api'),
-    Logger = require('./logger'),
+var Logger = require('./logger'),
     Robot = require('./robot'),
     Utils = require('./utils');
 
@@ -60,6 +59,8 @@ Cylon.api = function api(opts) {
     opts = {};
   }
 
+  var API = require('./api');
+
   this.api_instance = new API(opts);
   this.api_instance.listen();
 };