Updated MySQL docs and Ubuntu auto-install script

Created by: three18ti

Hello,

Please pull this one as it is based on the current master branch. Sorry for the confusion.

I have added some additional instruction for MySQL installation/configuration that I think others may find helpful.

Additionally, I ran into an issue where running ./resque.sh as root the first time creates a PID file /home/gitlab/gitlab/tmp/pids/resque_worker.pid owned by root and prevents the service from starting/stopping via init script as the gitlab user cannot write to the PID file. Prepending a sudo -u gitlab resolves this issue and makes this command look uniform with all the other commands in the install instructions.

Finally, I noticed in the instructions, the first line to install all the packages includes sendmail while the install script (https://raw.github.com/gitlabhq/gitlabhq/master/doc/debian_ubuntu.sh) does not. As such, I added sendmail to the install line in the debian_ubuntu.sh install script.

Thanks, Jon