Provide a list transpose() method to perform a matrix transpose

[jaccomoc]

The transpose method should perform the equivalent of a matrix transpose which swaps columns and rows.
E.g.:

[ [1,2], [3,4], [5,6] ].transpose()

results in:

[ [1,3,5], [2,4,6] ]

If any of the input lists is shorter than the others then null can be used to supply the missing values.