Added fpm systemd support for Jessie

[razvanphp]

I just tested this on my machine, but I don't know if it should be a new branch, as it probably doesn't work on Wheezy.

vagrant@aboutyou:~/php-7-debian$ sudo service php7-fpm status
● php7-fpm.service - The PHP 7 FastCGI Process Manager
   Loaded: loaded (/etc/systemd/system/php7-fpm.service; enabled)
   Active: active (running) since Wed 2016-01-13 15:48:46 CET; 12s ago
  Process: 21686 ExecStartPre=/usr/local/lib/php7-fpm-checkconf (code=exited, status=0/SUCCESS)
 Main PID: 21693 (php-fpm)
   Status: "Processes active: 0, idle: 10, Requests: 0, slow: 0, Traffic: 0req/sec"
   CGroup: /system.slice/php7-fpm.service
           ├─21693 php-fpm: master process (/usr/local/php7/etc/php-fpm.conf)
           ├─21694 php-fpm: pool www
           ├─21695 php-fpm: pool www
           ├─21696 php-fpm: pool www