Be able to chain injection steps and adds SteppedRampsInjection

I created a SteppedRampsInjection but before merging it, I also would like to finish the associated DSL. I haven't been able to find something satisfying:

```inject( steppedRampsTo(10000 users).thruRamp(100 users).over(30s).andNothingFor(10 seconds) )```

@slandelle, @pdalpra : any better proposition ?