add support for single column search

i needed filtering of single columns for a project, so i created these lines. I'm sure it's not the most beautiful code but as i'm new to ruby/rails and git i hope you will forgive me. probably this code is more an inspiration than a final solution. I would really appreciate if someone will support me to write some better code ;)

- the order of "search_fields" needs to be the order of the columns that are filtered - maybe that's not the best solution, would a new param be a better solution?