preparing a 2.14 release

Hi all,

there has been a recent breakage of darcs 2.12 build on stackage, and
fixing it would require bumping several dependencies upper bounds,
including base.

Rather than doing that, I find more economical to prepare a release of
Darcs 2.14. For this we need to fix the following:

* windows compilation (broken by Ben's encoding changes)
* make dependency on graphviz optional (hence the command `darcs show
dependencies`) (see http://bugs.darcs.net/patch1626 )

There are still bundles to review and tons of bugs to fix, but they
are not blockers for a 2.14.0 release.

Opinions?

Guillaume