move fee buffer off the income statement

[chadwhitacre]

Reticketed from gratipay/inside.gratipay.com#350.

cf. #8

The fee buffer shouldn't be on the income statement, because then we would need to "bury it in retained earnings" at the close of the year. We want to keep the fee buffer active in the balance sheet from year to year. It's basically an "allowance liability account." We'll see the fee buffer net on the balance sheet, and if we need change over time we'll run a separate report.

[chadwhitacre]

Done in #16.