added ability to get current users info

Added ability to get current users info without asking his name as like
this done here http://developer.github.com/v3/users/#get-the-authenticated-user

Author: coderaiser

github.js

@@ -80,6 +80,8 @@
       // -------
 
       this.show = function(username, cb) {
+        var command = username ? "/users/"+username : "/user";
+
         _request("GET", "/users/"+username, null, function(err, res) {
           cb(err, res);
         });