Ensure each ResultPersister (task-api) creates its own message sender.
* enables parallel sends
* persisters not closed by the user auto-close with a warning

Fixes #101
Merge branch 'master' of github.com:radek-macha/everBeen

Conflicts:
	task-api/src/main/java/cz/cuni/mff/d3s/been/taskapi/Task.java