custom sort function

This pull request is the application of a feature request by issue #258. 
I thought it was a good idea, but I am also OK to have it as a `wontfix`. 
Either way would like to see that issue closed :)

The idea is to be able to add a custom user function to the `.sort()` of a specific column. A concrete example, based on the issue where user needed to sort using Polish letters as `ń` would be this: http://jsfiddle.net/gUVFc/

Without this new possibility the `sort` would place that letter as last in alphabet.

fixes #258 
```
a.value.length - b.value.length
```

:wink: