Add check output to history

[ultrotter]

This can be useful for example to have a perspective on a flapping check. In term of storage history is only kept for 20 runs, so the extra space is not meaningful. Alternatives could be a configuration per-sensu or per-check to store this field or not, or simply if one has too verbose checks trim them or silence the output for them and just rely on status.

Just adding the output is a very small change (per #24) with most code changes being the result of compiling the proto.

Could we please discuss pros and cons, and see if we can get to include this functionality? Thanks!

Guido