Fixed bug 694 - python plugin mangles dataframe

This fixes bug [694](https://github.com/twosigma/beaker-notebook/issues/694).
Since slick grid template do not support rowspan we do not use it when any cell has either rowspan or collspan attribute larger than 1.
Also the logic for collapsing two header rows was incorrect.
please use a consistent coding style (no space between for and paren, ditto for if below)