Fixed initialization problem with null source, added new function to dinamically add new data, code refactoring

Hi,

I forked your nice project some days ago. I found two problem in the project for what i wanted to do with it. It crashes when you initialize a new gantt without passing any data and it does not allow to add new source data once initialized.
I've never developed a jQuery plugin before but i tried to follow what is written here http://docs.jquery.com/Plugins/Authoring
In my fork I refactored a bit the code to be more compliant, in my opinion, with jQuery specification and I developed also an addSource method to dinamically add new data.

Best regards.

Raffaello