Implemented Pascal's triangle

nit: you could make the output testable by separating the printing from the creation of the rows. Each row could be a Vec<uint> for instance.
If you wanted to get fancy you could also have an Iterator produce rows. Something like:

    // state of the iteration (we're just storing the current row number, as that's all we need
    // to produce a row of the Pascal Triangle
    struct PascalRowItem { cur_row: uint }

    impl Iterator<Vec<uint>> for PascalRowItem {
        fn next(&mut self) -> Option<Vec<uint>> {
             //...
        }