Added RelayState to response

[neilherbertuk]

As per SAML Spec, if RelayState is given, this must be passed back to the SP as is.

As Per official SAML docs (http://docs.oasis-open.org/security/saml/Post2.0/sstc-saml-tech-overview-2.0.html):
5.1.1

a binding-specific field called RelayState is used to coordinate messages and actions of IdPs and SPs, for example, to allow an IdP (with which SSO was initiated) to indicate the URL of a desired resource when communicating with an SP.

5.1.2.5

If the IdP received a RelayState value from the SP, it must return it unmodified to the SP in a hidden form control named RelayState. The Single Sign-On Service sends the HTML form back to the browser in the HTTP response.

RelayState can be used by SP to pass back after login the intended URL among other things such as tokens.

Not sure if storing this in session is the best approach but works well.

[steve-ks]

Thanks for your contribution,
I will test your code soon and would merge it then.

[neilherbertuk]

Removed pull request as I have a few other changes I would like to push in and need this to be somewhere else.