Added few CentOS related changes.
Optimized some commands, which won't require changing current working directory.

* Added default file for init script to allow configuration for the
runner
* Updated install area inside ReadMe.md
* Adapted the indention of the switch-case to match the rest of the
script

**EDIT:**

* Fixed description of line 17
* Edited defaultfile variables to match init.d-script variables

Upstart conf's should exist in /etc/init instead of /etc/init.d

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Ruby version ruby-2.0.0-p353 is required for gitlab-ci and gitlab-ci-runner works with this version, too. Result: interested parties do not think they need two versions of Ruby.

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

After the previous merge we can use the gitlabhq repo, also adding warnings for other operating systems.

Default ruby from ubuntu (1.9.3-p0) doesnt work.