Pre-compiling regex that removes control characters

Given that this regex might be used a lot in the lifetime of a given server (each request that uses get_argument for unicode strings) it seems sensible to store the compiled version of the regex.

I ran all the tests and apart from a couple SSL ones (23 due to old SSL) they all pass. I inspected the values myself and they seem to be the same.

Hope this change helps in achieving a little more performance from this already incredible framework.