Merge pull request #1030 from nremond/injections

Adds a "ramp rate" injection strategy
Don't want a Stream that might hold a reference on the body
Minor clean up
Log command line
Implicit duration conversion is done in the Simulation
Upgrade Slf4j 1.7.4 and Logback 1.0.10, close #1034
Should map onSuccess, not onComplete
Reorganize HttpCheckOrder
Properly name assertions failure (check is another API)
Minor clean up, use a case class instead of a Tuple
Minor clean up
Clean up circular mess between Configuration and ConfigurationBuilder
Expose warmUpUrl in gatling.conf, close #1035
Clean up and fix feeder: Singleton is by Feed, not by (Feed, number)
Merge @excilys/master into @nremond/injection_stream