automodsumm: created self.warnings at start of run

This was prompted by #552 - sphinx was crapping out when it issued warnings in a certain way, due to an error in how `Automodsumm` is run.  I'm not sure if this is because autosummary in sphinx changed since `Automodsumm` was written or just a mistake when it was first made, but either way, the warnings should now pass through correctly instead of triggering an exception.