Use Rubocop for coding style checks

- Generated default config by `rubocop --auto-gen-config`
- Fixed each issue in a separate commit and enabled the check (many issues were auto corrected using `--auto-correct` Rubocop option)
- Added `rubocop` call at Travis

Submitting to SLE-12 branch to avoid conflicts in the future and make maintenance easier.
better `|_opt, content|`