Feat: Write full message into short_message field

[valentin-krasontovitsch]

Following the principle of least surprise, we write messages completely into
thethe short_message field, even if they contain several lines.

As a consequence, we deprecate the full_message field, as it becomes
redundant.

Resolves #11

[valentin-krasontovitsch]

Since this is an API change (functionality of Write method is changed), it should probably be merged into a new v3 branch.

If you accept the merge request for more precise time stamps first, I'll be happy to rebase this guy on top of that.

[alexbusu]

Thanks for adding this. What if we can consider this PR a patch (like, for a bug fix), and let the merge in v2? 😄

[mariussturm]

@valentin-krasontovitsch thanks again for this PR! I would not simply remove the full_message field as it is still part of the GELF specification: https://docs.graylog.org/en/3.1/pages/gelf.html
Maybe a future version of GELF will sort this out but currently we should follow the specs. The full_message field is currently optional so why not just leave it there and use short_message like you described?

[valentin-krasontovitsch]

Okay finally coming around to looking at this again - sorry for the incredible delay...

Closing in favor of #34, which writes the whole message into both fields, period - hope this is what' desired ^^