Patch updated after review by Ezio Melotti.

To answer a common question that came up in the review: all atypical names and implementation details are there due to consistency with existing configparser code, e.g.:

 * readstring ~= readfp (no _ between words)
 * DuplicateOptionError ~= DuplicateSectionError (not Duplicated)
 * all exceptions use old style BaseClass.__init__ and not super()

API won't change so this has to remain that way. Exceptions may be refactored in one go at a later stage.