Stripe integration improvements

[arborrow]

1. Be able to associate a Stripe customer_id to a Polanco contact
2. Be able to associate a Stripe invoice->number with a donation ($charge->invoice; $invoice->number). When processing a first time invoice, create the donation. Generally the process would complain that the donation has not been selected; however, we want to allow it when it is the first invoice payment.
3. Consider pulling in start date, end date, donation install amount, terms (frequency) from Stripe invoice period_start and period_end fields