Multi vm tests

You can now execute your tests in multiple forked VM. Within a VM tests are executed sequentially, simplifying and making more reliable the output capture. This works well together with `testHideSuccessfulOutput`.
To activate this feature, forking must be activated.
The number of forked VM is set by `testNumberForkedJvm`. When set to more than one,  intra-vm parallel execution is disabled.

This is being discussed in 
  * https://groups.google.com/forum/#!topic/sbt-dev/E7gPr8iu9b4
  * https://groups.google.com/d/msg/sbt-dev/w7uli_-gQe8/FyhFYPhLX7sJ