capture WSGI-level errors

[untitaker]

from #19

Basically if Django/Flask itself crashes (or some wsgi middleware) we won't notice.

This is slightly related to #20 because we need to wrap the WSGI app last if somebody decided to use WSGI middleware in Django.

In Flask we can wrap Flask.__call__

An unsolved question is whether we want to attach basic request info for such events, and if we're better off doing the entire request parsing in the wsgi middleware at that point.