nixos: Add option services.cron.cronFiles

This allows you to configure extra files that should be appended to your
crontab. Implemented by writing to /etc/crontab when the cron service starts.
Would be nicer to use a cron that supports /etc/cron.d but that would require
us to patch vixie-cron.
stumpwm: update and refactoring

This commit updates the stumpwm to version 0.9.8. Futhermore, it
refactors the expression quite a lot:

* stumpwm has been moved from lisp modules to window-managers.
* stumpwm has been added to the window managers NixOS knows about, this
  enables the user to add stumpwm as a default window manager in his
  NixOS configuration like with Xmonad or i3.
* the package has been split into stumpwm and stumpwmContrib. This is
  due to the fact that development of stumpwm and its extension modules
  has been split into two repositories. As of today, the release is the
  last one before this split. This split into two packages only reflect
  those upcoming upstream changes already.

It is planned to make the addition of the extension modules voluntarily,
like with Xmonads option "enableContribAndExtras". Furthermore it might
be possible to add an option to compile stumpwm with clisp instead of
sbcl.
stumpwm: removed from lisp-packages.
fix fetchurl
fix sound in minecraft for pulseaudio users
qtcreator: Update from 3.1.0 to 3.2.0
haskellPackages.diagramsGtk: New expression
haskellPackages.engineIo: Update to 1.2.0
haskellPackages.engineIoSnap: Update to 1.0.2
haskellPackages.socketIo: Update to 1.1.1
blender: Update to 2.72
gtk-doc: update from 1.18 to 1.21 and add meta information
CompCert: update to 2.4
Update muPDF
Only update via external upstream information: nead to specify version format
Update sysdig
Update firejail
Fix the version
Update sodium to 1.0.0
Update SCons