Fix handling of column-menu and firefox issues

Fix the handling of column-menu whereby there were multiple copies of
the menu in the DOM.  Now only one exists and it will self-deregister
upon hide().

Also attempt to resolve the firefox issue with hidden columns taking up
calculated space in TDs even though not visible.

Fixes #57
Merge pull request #58 from jadrake75/hide-show-cols

Fix handling of column-menu and firefox issues