Update Dovecot mail_location value to store mail within a subdirectory inside of the user's mail home directory

As discussed on issue #105, here are the changes to one of the guides that sets the `mail_location` value to a subdirectory within the mail home directory. This prevents some of the issues mentioned on the official [Home Directories for Virtual Users](http://wiki2.dovecot.org/VirtualUsers/Home) Dovecot wiki page and also discussed on the mailinglist thread highlighted by these two posts:

* http://dovecot.org/pipermail/dovecot/2014-October/098379.html
* http://dovecot.org/pipermail/dovecot/2014-October/098400.html

There are additional changes to the [https://www.linode.com/docs/email/postfix/troubleshooting-problems-with-postfix-dovecot-and-mysql#virtual-domains-and-users](Troubleshooting Problems with Postfix, Dovecot, and MySQL) that are being addressed on PR #113 and a future PR.

P.S.

I didn't know what the official recommendation/practice was regarding merging in upstream repo changes prior to making a PR, so I made the assumption that I would need to do so in order for the changes to be accepted. Perhaps I'm just thinking of the CVS/SVN culture and showing fewer commits to a bugfix branch is the better way to go.