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.
r-XML: adding system-dependency libxml2
deadbeef: fix gtk3, use gtk2 by default, allow simultaneous
abbradar: update e-mail
cassandra: environment fix for cassandra shell script
alsa: Add multilib plugin support via "libs" entry in asound config.
alsa: Remove unused $ALSA_PLUGIN_DIRS support.
pulseaudio: Add support for 32bit alsa apps on 64bit systems.
python-scipy: update 0.12 -> 0.14

I successfully built all reverse dependencies, using this procedure[1]:

  $ nix-env -f . -qaP \* --drv-path | sort > l1
  (...modify package...)
  $ nix-env -f . -qaP \* --drv-path | sort > l2
  $ changed=$(diff l1 l2 | awk '{print $2}' | sort | uniq)
  $ nix-build $(for i in $changed; do echo -A $i; done)

[1]: https://nixos.org/wiki/Contributing
Merge pull request #4379 from flazz/master

cassandra: environment fix for cassandra shell script
Merge pull request #4368 from michelk/r-XML

r-XML: adding system-dependency libxml2
haskell-lzma-enumerator: disable builds of this package on 32 bit Linux

https://github.com/alphaHeavy/lzma-enumerator/issues/2
lib/licenses.nix: cosmetic
Clean up command-line tools a bit to allow us to use an impure
hdiutil (for an upcoming bootstrapped stdenv for darwin!)

Closes #4382
Update nixUnstable
perl-MooX-Types-MooseLike: add version 0.27
perl-Throwable: update to version 0.200010

The old version is no longer available for download
perl-Email-Sender: update to version 1.300014

The old version is no longer available for download
perl-Crypt-RandPasswd: update to version 0.06

The old version is no longer available for download.
perl-packages.nix: strip trailing whitespace
pdfjam: update from 1.20 to 2.08