Add a 'changeTop' option to the plugin.

I am working on a project that has a div above the floating table head. Sometimes the height of this div can change from the value I set for `scrollingTop`. 

With the changes in this pull request, one can simply call `$table.floatThead('changeTop', 500);` to change `scrollingTop` to 500px and update the location of the floating table header to use that value.