Make JWT the first argument for Elixir sign and encrypt functions

[tanguilp]

To make using pipes more natural with Elixir, I suggest that the first argument of signature and encryptions functions become the JWT to be signed or encrypted in the next version. It seems to me that it is a common practice in the Elixir community to have the subject of the operation as the first parameter of the function. It'd allow writing:

%{
iss: iss,
exp: now() + 30,
...
}
|> JOSE.JWT.from_map()
|> JOSE.JWT.sign(some_jwk)
|> JOSE.JWS.compact()
|> elem(1)