Injection API & DSL refactoring

With my changes, one can now write : 

```scala
val httpConf = httpConfig.baseURL("http://localhost:5000")

val scn = scenario("Scenario1").exec(http("get_request").get("/"))

setUp(scn.wait(4 seconds)
         .peak(10 users)
         .ramp((10 users) during (5 seconds))
         .constantRate((20 usersPerSec) during (15 seconds))
         .protocolConfig(httpConf))
```

The key of the implementation is the InjectionStrategy interface - simple but should cover all our cases. 

I'm planning to add two more injection strategies : 
* RampRate((10 usersPerSec) to (20 usersPerSec) in (1 minute))
* Dirac(???), cf http://en.wikipedia.org/wiki/Dirac_delta_function

I will work on the ScalaDoc if we agree that this is a good change.