Add a class to configure or disable the boot chime sound

This commit adds a `osx::global::boot_chime_volume` class that sets the `SystemAudioVolume` NVRAM variable to configure speaker volume at boot time so I can silence that infernal "bong" noise without having to remember to mute my laptop before rebooting it.

![6a0120a85dcdae970b0128776ff992970c-pi](https://cloud.githubusercontent.com/assets/979580/4587510/cbe9db10-502d-11e4-9442-33c81294611e.png)