title: Filter non-digits from string
body: <p>Using only swift code I cant figure out how to take "(555) 555-5555" and return only the numeric values and get "5555555555".  I need to remove all the parentheses, white spaces, and the dash.  The only examples I can find are in objective-C and they seem to all use the .trim() method.  It appears as though swift doesn't have this method but it does have the .stringByTrimmingCharacters method, but that only seems to trim the white spaces before and after the data. </p>