Support for multiple signature

[mod]

Hello!

I am wondering if you looked into
https://tools.ietf.org/html/rfc7515#section-7.2.1

it says:

   signatures
      The "signatures" member value MUST be an array of JSON objects.
      Each object represents a signature or MAC over the JWS Payload and
      the JWS Protected Header.

Signature are an array and we need this feature.
@potatosalad Did you knew about it, do you think we can support it on JOSE?

I could try to make a pull request but it will have significant impact on the code,
I am thinking to support verify with multi-signatures,
If the signature is an array we should verify all the signatures.

But maybe I am missing something and you have already a plan for this usage?

[potatosalad]

@mod The multiple signature support is still fairly lacking. I would like to add support for them at some point.