refactor(client): denodeify jwcrypto calls to reduce boilerplate.

@zaach - I was trying to figure out how to create signed JWTs and found a feature in P that could help use reduce boilerplate code with node-style callbacks. the P.denodeify function converts node-style callbacks to promise returning functions.
Merge pull request #1653 from mozilla/denodify-in-crypto

refactor(client): denodeify jwcrypto calls to reduce boilerplate.