Added shortcuts for zipping and combining streams as tuples.
Merge pull request #2 from sinicyn/feature/combine-as-tuple

Add shortcuts for zipping and combining streams as tuples.
Add repeatOn(impulse) combinator.
Add EventStreams.nonNullValuesOf factory method.
Add BiEventStream and TriEventStream.
Remove deprecated API.
Add emitOnEach(impulse) combinator.
zip and combine return (Bi|Tri)EventStream.
Add emitBothOnEach and emitAllOnEach combintors.
Update README.md
Reduce Javadoc warnings.
Move ListHelper to org.reactfx.util
Use maven range instead of gradle-style '+'.
Update README.md
Correct TriEventStream.flatMap().
Add flatMapOpt combinator.
Add Either type and or() combinator.
Override hashCode, equals and toString of Either.
Multiple updates to event sinks:

* Rename Sink to EventSink.
* Annotate EventSink with @FunctionalInterface.
* Add EventSink#feedFrom(EventStream) method.
* Add (Bi|Tri)EventSink and (Bi|Tri)EventSource.
Add EventStream.feedTo(EventSink) method.