Add initial support for ruby code style checking using the rubocop gem.
Add documentation for rubocop check.
Add config file setting to rubocop check.

And document it.
rubocop_check: don't pass the config file argument if file does not exist.
rubocop_check: only define this check if rubocop is installed.
rubocop_check: properly check for the existence of the rubocop gem.
rubocop_check: remove debugging statement
rubocop_check: turn off check for erb files.

Doesn't seem like rubocop properly supports erb files.
Merge pull request #61 from mnzaki/rubocop_check

Add Rubocop check