ObserveOn Fixes

- fix subscription leak (Composite+MultipleAssignment instead of just Composite)
- add unit tests
Merge pull request #602 from benjchristensen/observeOn

ObserveOn Fixes
UnitTest to assert thread safety of Executor Scheduler

- The Executor thread-pools hop threads when using observeOn so this tests that it is thread-safe when doing so.
- The test performs non-thread-safe mutations which should fail (or at least be non-deterministic) if memory visibility problems or concurrency is occurring.
Merge pull request #619 from benjchristensen/executorSchedulerConcurrencyTests

UnitTest to assert thread safety of Executor Scheduler
Properly wired up SynchronizedObserver within MergeDelayError operator
Merge pull request #628 from mattrjacobs/fix-merge-delay-error

Properly wired up SynchronizedObserver within MergeDelayError operator
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
Moved the descriptions of some operators to a new "Conditional and Boolean Operators" wiki page.
Merge pull request #639 from Netflix/docs

Moved the descriptions of some operators to a new wiki page.