Funding instruments endpoint

[mjallday]

The new dashboard has a cards/bank_accounts endpoint which returns both cards and bank accounts together.

In order to facilitate this I believe we need a new endpoint, let's keep it undocumented and call it funding_instruments to make things quick and simple.

This endpoint should be analogous to the transactions endpoint in that it returns all types of funding instruments.

[mahmoudimus]

This is a dupe of #229