Ruby style

I tried to respect some directions of ruby style by @bbatsov (https://github.com/bbatsov/ruby-style-guide)

 - Double quoted string with no interpolation and special symbols (ex: ```"new"``` -> ```'new'```)
 - Control flow statement with negative condition
   - Without ```else``` (ex: ```if !false``` -> ```unless false```)
   - With ```else```:
      ```Ruby
         #bad
         if !true
           #something not true
         else
           #something else
         end

         #good
         if true
           #something true
         else
           #something not true
         end
      ```
 - Unnecessary ```return``` statement
 - Unnecessary semicolon ```;```
 - ```Then``` in multiline if/unless block
 - Parentheses around conditional (ex: ```if (true)``` -> ```if true```)
 - Literal array syntax instead of ```%w``` (ex: ```['first', 'sec']``` -> ```%w(first sec)```)
 - Parentheses around empty arguments list (ex: ```def func()``` -> ```def func```)