Give ConfigParser the capability to set as well as read options, and to write

Give ConfigParser the capability to set as well as read options, and to write
a representation of the configuration state in .ini format that can be read
back in by a future read() call.  Thus this class is now a back end
for .ini editors as well as parsers.

This patch is complete and tested, but exposes a bug in the ConfigParser
implementation which I have not yet fixed.  Because case information is
discarded during parsing, the output of write() has its case smashed.

I wrote this for a SourceForge interface script called forgetool.
Documentation for the new entry points included.