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.
`option2Iterable` can convert `Option` to `Iterable` implicitly. `from(Iterable)` should be enough. Could you give an example to example why you need this one?