set VM's timezone to the host timezone

I would like the VM's time zone to be the same as the host machines.

This combines two techniques for getting & setting the time zone, and puts them in the Vagrantfile:

1. (get the `TZ` environment variable or look up the current timezone) [http://www.jaimegago.com/dynamic-guestrhel6-and-cousinshostos-x-time-zone-sync-in-vagrant-virtualbox-provider/]
2. (symlink `/etc/timezone` to a different timezone)[http://whomwah.com/2008/10/01/setting-the-timezone-on-centos/]