Added support for custom UL and LI classes.

[kostasx]

The use can now enter a custom CSS class for the UL and LI elements holding the tweets:

$('.tweet').twittie({
    'ulClass' : 'customClass',
    'liClass' : 'tweet'
});

[sonnyt]

@kostasx I am so sorry, I am just seeing this! This is a solid work. Can you please update the doc to include the info about the new config file?