Change ts sort algorithm to sortText

Changes tablesorter  algorithm from sortNatural to sortText. Fixes last comment by @meulie in issue #441

![screenshot from 2014-10-02 23 34 09](https://cloud.githubusercontent.com/assets/1818657/4498838/e389da2a-4a7b-11e4-807e-7455bfcdb7b6.png)