Allow publishers to specify additional payment details

[ericholscher]

We have a few publishers who want specific or interesting setups, which we should keep track of. Currently:

• Some prefer to be paid monthly, and some quarterly
• Some prefer to be paid in their local currency, and some in USD
• Some have other interesting setups (eg. getting ad credit), which we should document somewhere as well.

[davidfischer]

We do have the notes field but that will eventually not be enough. For the short term, I think we should figure out how common these different things are before we model for them. The ultimate solution might be letting publishers invoice us.