Allow multiple builds to run in parallel

Running independent build projects on the same build server leads may lead to long wait times until a project is built (e.g. has to wait for a long build that is currently running). It would be nice if several builds can run in parallel.

As running parallel builds inside a Continuum 1.1 project group may make no sense (dependent projects should probably not build in parallel?) it may make sense to allow one build process per project group.

Even on multi processor systems it will make no sense to run too many builds in parallel. Therefore configuring the maximum number of build processes is required.