Support for enveloping signature and getting the certificate chain

[finnsson]

I've added support for enveloping signatures and getting the certificate chain.

[finnsson]

I see that some check is failing. I'll update to the latest version of master and then make a new pull request.

[aodhan-domhnaill]

I merged this into https://github.com/russellhaering/goxmldsig/pull/69 because I'll need the changes