Add support for the "user" directive

Add support for the "user" Unicorn configuration directive. This allows
the daemon to drop privileges after it has been forked.
Update README for user directive
Refactor long 'define' line to multiple
Refactor `user` into forked_user and forked_group

Refactor the single `user` attribute into two separate attributes (one
for user and one for group). This makes it more clear as to how you
would specify a group for forked child processes. A log resource was
also added to the definition to alert the end user if they only specified a
group and not a user. Unicorn does not support dropping group privileges
only and requires that a user also be specified.