Rename CheckMethods to OnMethod

The module mainly provides the common hook for :def and :defs rather
than inspection logic.
Refactor CodeLength
Refactor EndKeywordAlignment
Rename ConfigurableNaming#check to #check_name
Refactor ConfigurableNaming

Not sure why the logic was unnecessarily compicated...
Rename IfThenElse to OnNormalIfUnless
Refactor NegativeConditional
Merge pull request #1195 from yujinakayama/refactor-mixins

Refactor cop mixins
Refactor StatementModifier