Provide an option to generically disable Split

This is roughly what I'm trying to achieve, but it's not passing the tests I added in this format.  Not quite sure why.

It passes your old tests, but not the new ones I've put in.

```
  1) Split::Helper ab_test should not store the split when Split generically disabled
     Failure/Error: ab_test('link_color', 'blue', 'red')
       ({}).[]=("link_color", "blue")
           expected: 0 times with any arguments
           received: 1 time with arguments: ("link_color", "blue")
     # ./lib/split/helper.rb:107:in `begin_experiment'
     # ./lib/split/helper.rb:191:in `start_trial'
     # ./lib/split/helper.rb:26:in `ab_test'
     # ./spec/helper_spec.rb:125:in `block (3 levels) in <top (required)>'

  2) Split::Helper ab_test SPLIT_DISABLE query parameter should also force the alternative (uses control)
     Failure/Error: alternative.should eql('blue')
       
       expected: "blue"
            got: "red"
       
       (compared using eql?)
     # ./spec/helper_spec.rb:113:in `block (3 levels) in <top (required)>'
```