RxScala: Add from method to turn an Option into an Observable

I don't know if this is a common enough occurrence to warrant adding, but I often find myself writing this boilerplate code when jumping between collections (or other sources of optional's) and Observable code. So I thought it might be an useful addition.

The emitted Observable's are made to match the Scala behavior when doing flatMap on the result.
I can run your unit tests without adding `from(Option)`