honor bSearchable option

[dansan]

I wanted to use this as a workaround for issue #12 with

"aoColumns": [
    null,
    { "bSearchable": false },
    null,
    ...
    ]

and noticed that the bSearchable option is ignored.

I have really only tested it only for the non-regex part :P

[pembo13]

This works well for me, and is very necessary if the queryset uses extra as filters on extra fields don't work.