Add eventlogging-reporter script

Adds 'eventlogging-reporter', a script which reports counts of incoming events
(both raw and valid) to StatsD. Streams are discovered automatically by walking
the /etc/eventlogging.d/processors directory.

Linux-specific, since it uses
<http://man7.org/linux/man-pages/man2/timerfd_create.2.html>.

Change-Id: I4cc35bd8e25a02b528920fdaf8817fd34e390aec