[Fix #577] A cop for requiring no parentheses in method definitions

This change introduces enforcing a method definition style without
parentheses like this:

    def columnsort array, size
    end

This change also reworks the DefWithoutParentheses cop and replaces it
with MethodDefParentheses with EnforceStyle: no_parentheses
Merge pull request #681 from skanev/issue-577

[Fix #577] A cop for requiring no parentheses in method definitions