Convenience methods for FA2

Hey guys, is anyone working on FA2 support already? Similar to the FA1.2 support [here](https://github.com/goat-systems/go-tezos/blob/master/rpc/fa12.go)?

We'd love to use this library to for [USDS on Tezos](https://github.com/stablyio/usds-tezos)

Let me know if there are any questions about this request or if there are no plans to support this so we can plan next steps!