Release beta rc4

I drafted this release a little bit earlier than usual to pull in one of the main bug fixes from #239 that blocks **beta-rc3** from working on *taurus*.

The other changes were already quite manifold and add good support to stability and code readability. Actually it is one of the releases with the most **fixes, code refactorings and added documentation** lines, well done!

As usual please review the [Changelog](https://github.com/ax3l/picongpu/blob/release-beta-rc4/CHANGELOG.md) and slap me for typos :)

@f-schmitt-zih @psychocoderHPC @PrometheusPi @Heikman 
Thanks for the great pulls you contributed and reviewed! :rocket: