RxJava interfaces

[nightscape]

I am a big fan of RxJava, RxAndroid & friends and would like to write a wrapper for the existing functionality of SensorLib.
Would you rather see this included as a module in this project (then I'd create a PR for it) or rather as a separate project?
The advantage of keeping it in this project is that it would be more easily discoverable and would be easier to keep up to date with breaking changes, the downside would be that it's additional (although quite simple) code to maintain in this repo.

[nightscape]

In #11 I coded an initial implementation to give you a better impression what that would look like.

[gradlman]

@nightscape If you wish to contribute directly to the project, you can contact me via stefan.gradl at fau.de. I am not sure if I find the time to check through this PR before the end of the week.