Merge branch 'benchmark' into 'master'

Benchmark

Contains:

* the benchmark, as discussed
* a test for the benchmark (without testing data loss)
* a protected static method in CompositeStage, to connect stages
* final declaration of all classes
* signal-bug fix in Merger
* new images in the download page
* EveryXthPrinter extends CompositeStage
* moved util.test

See merge request !28