Support JWKs with no algorithm

This patch was authored and released by @tanner0101.

Adds support for JSON Web Keys that do not specify an algorithm (#34).

JWKs are now stored directly in JWTSigners via the use(jwk:) method. When a request to verify a JWT is received that matches a JWK, both the JWT and the JWK's algorithm header is checked. A signer is then created on the fly to handle the request.