Added ability to customize field names, and disable mdc/context output.

Improved documentation.
Unit tests for field names and enable/disable mdc/context
Start using Travis CI
Add build status
Merge pull request #40 from philsttr/fieldnames

Customizable Field Names
Reformat
[maven-release-plugin] prepare release logstash-logback-encoder-3.1
[maven-release-plugin] prepare for next development iteration
Merge logstash/logstash-logback-encoder/master into philsttr/logstash-logback-encoder/master