Make node-logstash messages compatible with Logstash 1.2.x

In Logstash 1.2.x, message fields does not start with `@` anymore except
for `@timestamp` and `@version`.
`@fields` does not exists anymore (all fields are at the root of the
JSON message). This patch does not remove `@fields`, just rename other
`@` fields.
Merge pull request #38 from fasterize/logstash_1.2_compat

Make node-logstash messages compatible with Logstash 1.2.x