Add const mutator

* The const node has children, which need to be mutated
Add cbase mutator
Update flay threshold
Merge pull request #75 from mbj/cbase-mutator

Cbase mutator
Add N_TRUE and N_FALSE node helpers
Fix code formatting
Add binary connective mutator
Remove :not node

* The :not node is not used in Ruby 1.9+
Update flay threshold
Move true and false nodes above nil
Change connective mutator to mutate the operand boolean context
Fix whitespace
Add Mutant::NodeHelpers#n_not
Refactor Mutator::Node::Connective::Binary#mutate_operands
Refactor Mutant::Mutator::Node::If#mutate_condition
Merge pull request #76 from mbj/connective-mutator

Connective mutator
Fix mutator to not mutate the rhs in a boolean connective

* If the right hand side of the expression includes a method which
  raises or otherwise exits the control flow we cannot kill that
  mutation. However we can mutate the left hand side, and then
  mutate the whole node so it is negated; in the worst case both
  those mutations will be equivalent, but in the best case the rhs
  will return a boolean which will be negated.
Fix begin mutator to visit children
Correct regexp mutator in case of interpolation

Closes #67
Surface the output of rspec failures when killing noop mutants.