Support host

Added support for setting the `host` in each event.

In our scenario, we'll be running sumd on a central monitoring server. It will be probing multiple machines for various things (like whether http is responding in a timely manner). Each event that it generates will really tell whether http works on a *target* server, not the monitoring server. So we don't want `host` to contain the monitoring server's hostname.

With this patch, it is possible to choose the host for each task.

I haven't written any tests for this change. I'm not very familiar with python, yet alone its testing frameworks.