Jamesjames/exim on trade events consolidated

@calumhalcrow here are the final changes to fold exim trade events support into the consolidated trade events branch. thanks.
You can probably use the existing [parse_american_date](https://github.com/GovWizely/webservices/blob/master/lib/importer.rb#L62) here. Looking at the ITA importer, I think `process_date` can be replaced by calls to `parse_american_date` also. I'll take care of that though.