Add support for block APIs

[annybs]

Block APIs are defined here: https://github.com/edge/blockchain/blob/0a25bf472447afc4e2818b023fa7ab965ad5996e/src/api/index.js#L45-L52

These are not essential for current work on CLI wallet, but would be useful to add later, defining xe-utils as the primary client library and making it usable by other applications.