title: Convert charset name to NSStringEncoding
body: <p>Given a charset string, such as "utf-8", "iso-8859-1", "us-ascii" etc, is there any built-in way to get the appropriate NSStringEncoding in Cocoa?</p>
  
  <p>Right now I'm looking at just building a NSDictionary containing a canonicalized version of the name mapped to the NSStringEncoding, then having a lookup mechanism that canonicalizes the input in the same way.  But is there really no way to get NSUTF8StringEncoding given the string "UTF-8", etc?</p>