+pro,ben add akka-bench-jmh, enable running jmh benchmarks form sbt

This has one TODO still open, but in case you want to give JMH a shot this week locally - it works already.

Working integration with JMH.
Details in commits - it's now runnable by just going to the `akka-bench-jmh` (better name ideas?) project and running `run [any jmh options]`. Example: `run -i 3 -wi 3 -f 1 .*actor.*`, for understanding the options simply pass in `-h`.

todo:
- [ ] The actual run works fine, but the preparing requires 2 compile runs: `clean; compile; compile` - because during the first compile jmh generates java sources from scala class files, and only on the second compile run these sources are available and can be compiled to classfiles which the benchmark actually picks up. I believe this is a workaround for annotation processing. I want to solve this - so it automatically compiles the generated sources. //cc @jsuereth how can I do this easily?
- [ ] make this a plugin == do not polute plugins with normal dependencies (on jmh)

// btw "why not a loop but JMH": http://hg.openjdk.java.net/code-tools/jmh/file/f2e982b7c51b/jmh-samples/src/main/java/org/openjdk/jmh/samples/JMHSample_11_Loops.java

Eventually will resolve #15180