Naive schedulers implementation
Added ScheduledExecutorServiceScheduler
Added to Schedulers
More tests
Headers
ObserveOn/SubscribeOn unit tests
Some documentation
Documenting code
renamed tests
Extracted ScheduledObserver as a separate class
Reimplementation of Concat, improved handling of Observable<Observable<T>>.

The old version required all of the Observable<T>s to be generated and buffered
before the concat could begin.  If the outer Observable was asynchronous, items
could be dropped (test added).  The new version passes the test, and does the
best job I could (after examining several possible strategies) of achieving
clear and consistent semantics in accordance with the principle of least
surprise.
Incorporate review feedback.

Also restore the errant change to settings.gradle.
Merge branch 'upstream' into issue202
Rename Subject to PublishSubject
Introduce Subject as analogue to RX ISubject
Implement RepeatSubject
Extract UnsubscribeTester to top level
Beef up UnsubscribeTester
Beef up UnsubscribeTester some more
Add unit tests to PublishSubject