Added "init.d" service script for Debian-based Linux.

Compatible with "/etc/default/gogs" configuration file for adding or overwriting values.
Based on "/etc/init.d/skeleton" and tested on Raspbian 2014-06-20 (Debian 7.5 [Wheezy] ARM)

[TO FIX] For some reason, it's necessary to remove the value of `RUN_USER` from the `app.ini` configuration file because the following error occurs:
`2014/09/28 18:12:53 [setting.go:182 NewConfigContext()] [E] Expect user(git) but current user is:`