Add slow request logging incl. recording of segments (based on kamon).

To test this:
```
$ sbt stage
$ $ ./target/universal/stage/bin/play2-slow-request-logging -J-javaagent:`pwd`/etc/aspectjweaver-1.8.4.jar
$ curl http://localhost:9000
```

should produce a log message like

```
2014-12-13 22:12:30,656 SLOW "GET /" (139 ms), segments: op1 (67 ms), op2 (69 ms, numItems: 2)
```