Fixed mesos-style.py errors.
Re-enabled low level scheduler tests.
Made doxygen recursive to include nested files.

Running doxygen without the 'recursive' flag enable didn't capture any
source or include files as we only point to include/ and src/.

Review: https://reviews.apache.org/r/24430
Added 'process::inject::exited()' to simulate exited events in tests.

Review: https://reviews.apache.org/r/20504