update debian init script to be more robost

Add --retry when stopping the agent so that after TIMEOUT (default=5) the process is killed. Previously there was no check at all on wether or not the agent actually exited.

Adds configurability via the standard /etc/default/newrelic_plugin_agent files.
Merge pull request #234 from trbs/deb_init_timeout

update debian init script to be more robost