Add typ, cty fields in the JWT header

This patch was authored by @Maxim-Inv and released by @tanner0101.

Adds typ and cty fields to the JWT header (#25, fixes #5).

• Add typ header as parameter making "JWT" the default.

• Allow setting cty during JWT signing.

let signer = JWTSigner.hs256(key: keyData)`
let token = try signer.sign(payload, cty: "twilio-fpa;v=1")