Added a simple PoisonPill capability to the TaskService

The PoisonPill allows you to write producer/consumer patterns with the
TaskService as the consumer and signal the end of work using the
PoisonPill task.
Updates to the TMSPackager to be able to start packaging immediately without waiting to collect all the tasks up front.
Merge branch 'master' of https://github.com/gwaldron/osgearth