Added op:join to concat objects with separator between elements.
Javadoc improvements:
* consistent nomenclature
* consistent/standard javadoc formatting
* more-precise descriptions
* timer() marble digram & wiki link
* delay() marble diagram & wiki link
Merge pull request #607 from Netflix/docs

Javadoc improvements:
Merge pull request #604 from akarnokd/StringObservableOps1

Added op:join to concat objects with separator between elements.
Operation Timer 3.0
CustomReplaySubject to support various replay() operators.
Operation: Replay additional overloads
Merge pull request #609 from akarnokd/OperationTimer3

Operation Timer 3.0
Merge pull request #612 from akarnokd/ReplayAdditionalOps

Replay additional overloads
Copied the code from OperationMerge
Merge pull request #615 from abersnaze/issue614

Copied the code from OperationMerge to fix synchronization
replay() and timer() javadocs: diagrams & wiki links
Merge pull request #617 from Netflix/docs

replay() and timer() javadocs: diagrams & wiki links
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