fixes #53 (files produced need repair)

The problem was that when we build the final object tree for marshalling
in XML, we use make([]Col, MaxCell) and then use append on the resulting
array. This has the undesired effect of initializing an array with a
number of empty cells and then appending to that array. The result is that
the final XML contains a bunch of empty column definitions before the
real columns start.
Merge pull request #60 from ilowe/master

fixes #53 (files produced need repair)