Remove bad IPN support

It doesn't event fit the format of delivered events.  Did they
used to be different?
Add event retrieval support

Completely untested
Populate customer id in charge record
Add customer retrieval and populate subscription
Fix guard denying empty fields for event/customers
Fix event retrieval to use GET instead of POST
Fix event parsing into records
Add invoiceitem record support
Add invoiceitem retrieval support
Accept non-implemented stripe types

Just return that it's not implmented with an info dump.

Also, stop existing_atom'ing the stripe types beacuse there is
a limited number and we don't have them all enumerated in the code (yet?).
Add missing customer field to invoiceitem record
Add plan_id type

Cosmetic type change only.  No possible operational impact.