don't promisify non-functions, duh

lib/core.js

@@ -10,7 +10,9 @@ var util = require('util');
 function promisifyAll(obj) {
   var aobj = {};
   Object.keys(obj).forEach(function (key) {
+    if ('function' === typeof obj[key]) {
       aobj[key + 'Async'] = util.promisify(obj[key]);
+    }
   });
   return aobj;
 }