Fix for tables with multiple rows and colspans in headers

Hi. In a recent project I had a table where some of the columns had to be logically grouped.
Typically this can be implemented as a table with header containing multiple rows with some of the cells spanning multiple columns (see problem demo below for example).

There are 2 issues preventing current implementation from doing this:
1. sorting breaks when there are multiple rows in header.
2. sorting breaks when there are "colspans" in td's of the header.

This pull request contains 2 commits addressing these issues and allowing to create such tables.

Demo, showing the problem (open test.html and try to sort tables): https://github.com/EugenyLoy/bootstrap-sortable/tree/colspan_fix_problem_demo
Demo, showing solution: https://github.com/EugenyLoy/bootstrap-sortable/tree/colspan_fix_wip

Implementation tested on IE 8, IE 10, FF 25 and Chrome 31.