Add a SYSV init script to the deb package

Debian Wheezy does not natively support Upstart, so add a SYSV init
script which is only activated if necessary via the deb package post
install script (marathon.postinst).
Marathon build assets should be removed on clean

When packaging, we don't have logic to deal with multiple build packages
left over in the marathon source directory. This helps with that issue
and will make sure our builds are as clean as possible.