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.