Including Button's __init__ css_class argument on form template

Hi, 
First i'd like to thank you for all the great work and effort you have put in this distribution. It's a great time saver and I really love what you have done. That said, i've found an small issue with the custom classes of buttons (those passed through the css_class argument in __init__ method). Currently the code ignores that argument (none of the classes passed as arguments gets to the html) and i've found that when you override the deform's form.pt template you don't include, in the fields.buttons block, the button's css_class content. That impedes user from giving custom classes to form buttons, i've added a small snippet that fix this issue, please review it whenever you have the time and if you have any comments i've be glad to be of any help.