Add ExecutionContextScheduler for Scala ExecutionContext

[zsxwing]

As per #77

[samuelgruetter]

Are you sure that this is "the right way" to do it? Can you answer the question I asked in #77 (comment)? And somehow, Java and Scala are compatible, can we exploit this better? The following compiles:

val ec: java.util.concurrent.Executor = scala.concurrent.ExecutionContext.Implicits.global

And do we also have to care about ExecutorServices?
I'd prefer adding nothing than something we don't understand...

[zsxwing]

Answered in #77

[zsxwing]

@samuelgruetter could you revisit this PR? I added unit tests to verify the thread name. For ExecutionContextExecutor, since it's already a java.util.concurrent.Executor, we don't need to wrap it.

[zsxwing]

Updated the examples.

[samuelgruetter]

LGTM