Closes #102

- use start-stop-daemon's chuid feature insted of su
  - using `su -c` breaks if the git user has no usable shell
  - this fixes #1025
- put --test before --exec, instead it gets passed to gogs
- set cwd via --chdir

Implies 2 running child, so the `stop` command usually needs a `KILL` instead of a `TERM` to close properly so I reduced it to 1 second to get quickly with the `KILL` signal.

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: