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.
Add extended trace converter to syslog formatter

Adds support to the syslog formatter but does not add it to the default
pattern, which currently omits stacktraces.
Merge pull request #261 from nicktelford/feature/logging-exception-packageinfo

Add logback converter for extended stack traces