Add audio support in chroot, by forwarding audio to cras (Chromium OS audio server).

 - Bind mount /var/run/cras.
 - Compile and install cras ALSA plugin, and set it up as default device.
 - Remove pulseaudio if possible, then prevent it from being installed.

*Pulseaudio:*
From what I tested, pulseaudio is not a "strict" dependency of any package crouton pulls in, so I think it is safe to remove it.

There is a way to forward the audio from pulseaudio to cras, but I don't think it brings much (since cras does mixing already), and adds some complexity. I will put instructions on the wiki if someone has a compelling reason to use pulseaudio.

*Volume:*
Volume can be changed in `alsa-mixer`, but Chrome OS is not aware of these changes, so when you press, say, Volume Down, it decreases the volume based on the last volume set by Chrome OS