Previously header ids were autogenerated by the writers.
Now they are generated (unless supplied explicitly) in the
markdown parser, if the `header_identifiers` extension is
selected.
In addition, the textile reader now supports id attributes on
headers.
* MakeManPage.hs has been transformed into
man/make-pandoc-man-pages.hs.
* There is now a cabal stanza for this, so the dependencies are
handled by cabal.
* Special treatment in Setup.hs ensures that this never gets installed;
it is built and used to create the man pages.
* Setup.hs cleaned up.
