Feature: Groups!

Groups can now be added by either the API or a text file.

API example:

````javascript
var urls = [
    {
      group: 'Circles',
      url: 'http://leanin.org/circles/'
    },
    {
      group: 'Circles',
      url: 'http://leanin.org/circle-login/'
    },
    {
      group: 'Circles',
      url: 'http://leanin.org/circle-faqs/'
    },
    'http://leanin.org',
    {
      url: 'http://leanin.org/team/'
    },
    {
      group: 'News+Inspiration',
      url: 'http://leanin.org/news-inspiration/'
    },
  ];

var shot = new Screenshot(urls, options);
````

Text file:

````
http://leanin.org

Homepage
http://leanin.org/about/

News+Inspiration
http://leanin.org/news-inspiration/
````