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.
fix sound in minecraft for pulseaudio users
Merge pull request #4388 from NixOS/minecraft-sound

Minecraft sound with Pulseaudio
Re-add bower/bower2nix, regenerated node-packages-generated.nix
at: update from 3.1.15 to 3.1.16
btrfsProgs: update from 3.16.1 to 3.16.2
cifs_utils: update from 6.3 to 6.4
bluez5: update from 5.23 to 5.24
atom: update from 0.129.0 to 0.135.0
dos2unix: update from 6.0.6 to 7.0
ddrescue: update from 1.18.1 to 1.19
cupsBjnp: update from 1.2.1 to 1.2.2, potentially fixes CVE-2014-2856
couchdb: update from 1.6.0 to 1.6.1
byobu: update from 5.68 to 5.87
bitcoind: update from 0.9.2.1 to 0.9.3
dnsmasq: update from 2.71 to 2.72
ethtool: update from 3.15 to 3.16
gmime: update from 2.6.19 to 2.6.20
traceroute: depends on linux headers.
Nixpkgs manual: Mention xz support

Issue #4393.