Stop writing threads when ExecutorService is shut down.

Without the !Thread.interrupted() condition, the thread continues
working even though it was interrupted by shutting down the executor
service. Didn't know that. See
http://docs.oracle.com/javase/tutorial/essential/concurrency/interrupt.html.
Testing many tailers waiting on the same log watch.

The test doesn't uncover any problem but is a nice addition to the
suite.
Merge pull request #16 from yurloc/thread_tests

Thread tests