Correcting spelling of "threeshold" configuration item

Hey, I fixed a misspelling for the threeshold configuration item.  I thought it might have just been a british spelling, but upon looking it up I don't think "threeshold" is a valid spelling.  http://www.merriam-webster.com/dictionary/threeshold

I went ahead and added some compatibility for the old option, so if you merge this pull request all your current configuration with the wrong spelling should still work no problems.  

Thanks!