Display and events handling fix for tables with sorting disabled for specific columns

Current implementation does not handle columns with sorting disabled with `data-defaultsort='disabled'` well enough.

First of all when user howers mouse over header it changes cursor making it the same as when howering over sortable column headers. This confuses user as it is appears like he/she can apply sorting to the column when he/she actually can't.
Second - when user presses header, `'sorted'` events got fired up even though there were no sorting happened.

This pull request fixes these issues.

Problem demo (open `test.htm`): https://github.com/EugenyLoy/bootstrap-sortable/tree/disabled_columns_soring_fix_problem_demo
Fix demo: https://github.com/EugenyLoy/bootstrap-sortable/tree/disabled_columns_soring_fix_wip