Added LogstashAccessFieldNames for consistency, changed "message" to "@message" on LogstashFormatter and created an abstract formatter to avoid code duplication
Fixed tests failing with the change of "message" to "@message"
Set the default field name of the message field back to 'message' for LogstashFormatter,
but leave it as '@message' for  LogstashAccessFormatter.

This ensures backwards compatibility for the default field names for both formatters.
Add Edu Garcia to contributors
Add note to README.md about logstash access field names
Add some minor javadoc
Merge branch 'Arcnor-master-53' for pull request #53