Integration testing

[pbrisbin]

We lack any good way to test things end-to-end: e.g. to assert on the result of some notify(With) call given some scenario. This makes it hard to get a regression test on #31 for example, which led to us not really fixing it.

I'm thinking about something like sticking an optional IORef in Settings that, when present, gets the Events appended to it instead of actually send to Bugsnag. This could even be useful to end-users who want to test Bugsnag-related paths in their own applications.