Provide Observable.timestamp(Scheduler) to be used in the tests.
Merge branch 'issue750' of github.com:vigdorchik/RxJava into 751-merge

Conflicts:
	language-adaptors/rxjava-scala/src/test/scala/rx/lang/scala/ObservableTest.scala
Merge pull request #876 from benjchristensen/751-merge

Manual Merge of #750