Remove shaded jackson

 Do not shade jackson. Instead, include it as a dependency.

Shading jackson was causing the jackson serializers to ignore annotations
on objects (since the classes of those objects used the 'real' jackson annotations
(under `com.fasterxml.jackson.annotation`),
but the shaded jackson classes were expecting the shaded classes
(under `net.logstash.logback.encoder.com.fasterxml.jackson.annotation`).


I accidentally branched this from the changes in my other pull request ( #46 ), so those changes are included as well.

The only changes related to jackson are to the pom.xml file.