Add the rest operators to RxScala

[zsxwing]

This PR added the rest operators to RxScala. However, I think some operators in rxjava-math, rxjava-string , such as min, max, average, are worth to add to RxScala. What do you think? @headinthebox, @samuelgruetter

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[headinthebox]

If we put them in a different package as extension methods I'm fine. Then you can bring them into scope them as you wish.

[headinthebox]

Thanks @samuelgruetter and @zsxwing
I'll review this in depth tomorrow (sorry) Friday morning.

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[zsxwing]

I gave up the idea that converting Map[K, Seq[V]] to Map<K, Collection<V>>. It's much more complicated than I thought. Directly implementing toMultimap in RxScala looks much easier.

[zsxwing]

I will rewrite toMap in another PR as it's a breaking change.

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[zsxwing]

But then I think RxScalaDemo should clearly explain that obs.subscribe(onNext), obs.foreach(onNext), and for (e <- obs) onNext(e)) are exactly the same.

Added subscribeExample for this. I added it at the beginning of RxScalaDemo so that people can easily notice it.

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[zsxwing]

Overrode the last commit to fix the typos.

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[samuelgruetter]

LGTM. Thank you @zsxwing!

[headinthebox]

Thanks @zsxwing .

@benjchristensen can you merge this, then I can make a pass over it so see if there is any fine tuning left.