import thresholds from thresholds.c
Merge branch 'feature-riemann-threshold' of github.com:pyr/collectd into feature-riemann-threshold
Add threshold checks for riemann output.
Store the avl tree within collectd core.

We need this if we want modules to share access to the threshold
avl tree.
reuse main avl tree
Merge branch 'master' into feature-riemann-threshold
make threshold checking optional
document new options
Merge pull request #518 from pyr/feature-riemann-threshold

Feature riemann threshold