Add check output to history

[ultrotter]

I filed this as sensu/core#27 and it has an associated pull request sensu/core#24 which is really a one-liner plus a protobuf recompile.

Feature Suggestion

The idea is to keep the scripts output in the history of the check execution, for debugging or to share extra information between the check and downstream from sensu. We already have the output of the last execution, and this shouldn't change the profile of the db size much, as there is only a limited number of entries in the history.

Possible Implementation

sensu/core#24

An alternative would be adding the field per the above, but also adding a per-check config on whether to keep the history or not.

Context

We are trying to have access to a bit more history of the output of checks.