Add variable mutator

* This branch will add a mutator that handles gvar, cvar, ivar and
  lvar nodes.
Add Mutant::Mutator::Node::Variable

* Rename lvar mutator to handle all variable usage
Refactor Mutant::Mutator::Node::Generic#dispatch guard clause
Update flay threshold
Merge pull request #70 from mbj/add-variable-mutator

Add variable mutator