changes prompted by discussion of issue #31

Hi,

I know our discussion didn't reach any firm conclusions about what changes (if any) should be made, but I think this pull request adds some useful features without breaking any existing functionality.

What I've done is added a config param that checks if the airbrake server/plugin configuration are correct on startup and if not, sending notifications is disabled. If this config param is omitted, there are no changes in behaviour.

I've also added a means of enabling/disabling the sending of notifications at runtime. See the changes in `README.md` for a more detailed description of these features

Thanks.