Mutation API rework

[mattkrick]

The mutation API should be more functional. We can make it more reduxy by providing an optimistic and response callback.

Additionally, a general problem with cashay-- when i accept an invitation i want both the new team member and a JWT, which would be a custom mutation payload. But, since there is no query that uses the JWT, cashay will never generate a request for it. Granted, we're doing something really weird by working off the the mutation's promise, so it's an edge case for sure. With the new mutation API, we should be able to add additional fields that we want to fetch.