title: Trim any illegal characters from NSURL before writing to disk
body: <p>I would like to write a string to an <code>NSURL</code> which may contain characters such as <code>/</code> or <code>:</code>, which are not supported by the file system. Is there any convenience method to trim these characters? Or maybe a reference containing all illegal characters, so that I can write such a method myself?</p>