cartodb_url param on CartoDB instantiation

[busla]

Since I host CartoDB myself I needed to allow for an additional parameter cartodb_url
and change resources.json to reflect the change.

Creating a client can now be run with:

var client = new CartoDB({
  user: secret.USER, 
  api_key: secret.API_KEY,
  /* e.g. http://myusername.cartodb.com */
  cartodb_url: ''
});

master...busla:master

You want a PR?

[impronunciable]

A pr would be nice since you can test your own instance. Otherwise let me know and I'll make the changes.