move info signal handling into runner; make optional by way of accessor.

Here's a solution for #207. Moves the info signal handling to the runner and makes it a toggle, detected by default.

Here's some example code that exhibits how it might be used, other bits are immaterial and just add context, but might be familiar if you remember our conversation about suite hooks.

```ruby
class MiniTest::Unit::ProvisionedRunner < MiniTest::Unit
  def _run_suite(suite, type)
    begin
      suite.before_suite unless suite.test_methods.empty?
      super(suite, type)
    ensure
      suite.after_suite unless suite.test_methods.empty?
    end
  end
end

MiniTest::Unit.runner = MiniTest::Unit::ProvisionedRunner.new
MiniTest::Unit.runner.set_info_signal = false
```