Make NewThreadScheduler create Daemon threads

This matches the behavior of Schedulers.COMPUTATION_EXECUTOR and Schedulers.IO_EXECUTOR.

See https://groups.google.com/forum/#!topic/rxjava/Qe1qi0aHtnE and https://github.com/Netflix/RxJava/issues/431#issuecomment-30767610
Fix non-deterministic unit test

- there is no guarantee for how many threads Interval will use so useless to assert anything on it
Merge pull request #631 from benjchristensen/NewThreadScheduler-Daemon

Make NewThreadScheduler create Daemon threads