OS X installer can run Jenkins as user "jenkins"

User has to choose a custom installation.
There are two choices:
- Start at boot as "daemon" (enabled by default)
- Start at boot as "jenkins" (disabled by default)

If user disables the first and enable the latter, the installer
will create a new user "jenkins" and new group "jenkins"
(if they do not exist), chown JENKINS_HOME to jenkins:jenkins
and set up launchd job to run Jenkins as "jenkins"

Also fixed some permission errors that PackageMaker complains about
when building the package.

Also PackageMaker xml file reformatted to be more readable