[WIP] Rerun suite after code generation

I'm putting this here for feedback. Automatically reruns the tests after class or method generation

![rerun](https://cloud.githubusercontent.com/assets/237866/4035941/0f38bc18-2c95-11e4-97b8-4f27b1b2f3a2.gif)

2 strategies are used to do this:

Passthru - the fallback for non-POSIX systems, this starts a new process for each rerun. This could in theory lead to tens of processes; all will terminate eventually.

Pcntl - on POSIX systems the current process is abandoned and a new one started in the current process space - there is no overhead in this case.

To do:
- [x] Initial implementation that works on Unix
- [x] Some strategy for testing in Behat
- [ ] Check it doesn't explode on other platforms (Windows)
- [ ] Some configuration - should it default to on or off?
- [ ] Incorporate #417 and #481 if they get merged 

 
missing example when first supported child is 2nd runner?