rtree2

I've made a **new version** of *rtree* at https://github.com/davidmoten/rtree2. It has a different artifactId *rtree2* and different package names so can co-exist with *rtree* if required.

Instead of supporting RxJava 2 API I've dropped the reactive API completely and now search methods return `Iterable`s. You can then wrap the return with whatever reactive library you like. For instance for RxJava2:

```java
Flowable.fromIterable(tree.search(rect))
```
One really interesting consequence of dropping the reactive approach in the API is that performance went down about 20% on search benchmarks. It seems that high request fast paths were performing really well and converting to Iterable which is like a request 1, request 1, etc Observable/Flowable is not as fast. Anyway I think the flexibility benefit and reduced dependencies is worth the performance hit. I'll keep looking for opportunities to bring perf up again.

Note also that I've dropped serialization because it's a big maintenance pain. You can do your own serialization and deserialize using bulk packing kindly contributed by @ambling in #67.

I've released rtree2 0.9-RC1 to Maven Central which you can try out.