Use a proper URL manipulation library instead of string concatenation

[akiroz]

I couldn't find this issue mentioned anywhere so I'm opening one... do correct me if there's already an issue for this.

String concatenation is used in most places to handle URLs. As a result, this code will cause a cryptic error message when using skygear:

skygear.config(
  'https://myapp.skygeario.com', // <- missing trailing slash
  'a22c50e0acb3447c81f9c5756e7331e7'
);