Autoscaling: Fix case where target < current, but target is too low.

This can happen when workers are provisioned manually.
Merge pull request #574 from metamx/srms-hopscotch

Autoscaling: Fix case where target < current, but target is too low.
change default merge size to 500mb instead of 1g
Merge branch 'master' of github.com:metamx/druid
Better task log errors.
Merge pull request #575 from metamx/task-log-errors

Better task log errors.
fix doc mistake