Install Kafka into a version specific directory

To facilitate upgrading a Kafka node it’s somewhat necessary to install
each version into each own directory and then maintain a symlink to the
currently installed version. If there had been an actual package to
install that would’ve simplified things, but alas there’s none at the
moment that’ll cover all supported platforms, so this is the next best
thing.

Furthermore the home directory of the `kafka` user has been changed to
`/var/empty/kafka` rather than the install directory. Why this particular
directory you might ask? Well, I inspected /etc/passwd on CentOS 6.5 and
noticed that some (service) users used /var/empty/<service> as their
home directory, so I thought why not?
Extract variables in `_install` recipe to helpers
Remove comment in `_coordinate` recipe
Merge branch 'version-specific-install-dir'

Closes #16.