Enable AutoTooltips plugin to work with header cells

Added functionality to AutoTooltips plugin to optionally work for header cells in addition to data cells.

1. Added config option to enable AutoTooltips for header cells (and option to disable it for data cells)
3. Added tests for AutoTooltips to test current and new behavior
4. (Optional) Fixed slick.grid.js unit tests

Needed this for a project.  If you specify a `tooltip` option on the column definition, then the behavior will be ignored.