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
Improved framework rate limiting by imposing the max number of outstanding messages per framework principal.

Review: https://reviews.apache.org/r/24343
Fixed a bug in slave when launching container.

Review: https://reviews.apache.org/r/24440
Reverted a refactor that broke the build.