- Template no longer inherits from unicode.

- SafeTemplate is removed.  Now Templates have both a substitute() and a
  safe_substitute() method, so we don't need separate classes.  No more
  __mod__() operator.

- Adopt Tim Peter's idea for giving Template a metaclass, which makes the
  delimiter, the identifier pattern, or the entire pattern easy to override
  and document, while retaining efficiency of class-time compilation of the
  regexp.

- More informative ValueError messages which will help a user narrow down the
  bogus delimiter to the line and column in the original string (helpful for
  long triple quoted strings).