Re-apply felixclack's pull request 227

I've re-applied felixclack's gh-227 with various changes to get the tests passing.

I'm using this on an internal test suite with the most recent version of cucumber and it works fine in addition to cutting the run time in half (selenium tests).

I'll take a stab a fixing the jruby issues this weekend, not sure if there is a better way to get the example data out of Cucumber/Gherkin.

It'd be great if anyone else could give it a try, I'd love to get this merged.

``` ruby
gem 'parallel_tests',
  :git => 'https://github.com/silas/parallel_tests.git',
  :branch => 'group-by-scenario2'
```