DataFrame_repr_html changes according to #772 discussion.
cc @ellisonbg

Implemented changes as discussed in #772.
Not so crazy on the scrollable div. For now i left the div max sizes as they were, could use suggestion on _good_ values.

I don't use the notebook often, what i noticed just now is that in combination with the IPython notebook `get_terminal_size()` returns the size of the terminal where the notebook was launched, not the size of the notebook display area. Don't know if there is a way to get the notebook size.
