Modified parsing of format strings, so that we always return

Modified parsing of format strings, so that we always return
a tuple (literal, field_name, format_spec, conversion).

literal will always be a string, but might be of zero length.
field_name will be None if there is no markup text
format_spec will be a (possibly zero length) string if
  field_name is non-None
conversion will be a one character string, or None

This makes the Formatter class, and especially it's parse()
method, easier to understand.

Suggestion was by Jim Jewett, inspired by the "tail" of an
elementtree node.

Also, fixed a reference leak in fieldnameiter_next.