Sweeping improvements to riemann-riak

All events will flip to state critical if Riak is down. Modularized
code. Added error checking for riak-admin status failure.
riemann-health: switch to 1-minute load averages.
Version 0.1.9
riemann-riak: du follows symlinks.
Merge branch 'master' of github.com:aphyr/riemann-tools
0.2.0