Add logback converter for extended stack traces

Dropwizard prefixes logged stack traces and optimizes them for sleepy ops folk. However, "extended stack traces" made available by Logback are not formatted.

These traces add package information to each frame so you can trace which JAR the code was executed from.

This patch adds a converter for these extended stack traces, which can be enabled in your log format by using "%xEx" in place of "%ex".

Since this is quite useful, I've also enabled this by default.