Add support for signature.privatekey for saml-sp-remote

Related to conversation at https://groups.google.com/forum/#!topic/simplesamlphp-dev/AyhDq1kzGD8
Regarding SAML2 IdP key rollover should allow transition per SP

This patch allows a signing key to be specified within saml2-sp-remote allowing one to pursue a signing certificate transition on the IdP, even if one or more of your partners is unable or unwilling to transition.

This is especially useful for large scale IdP's with multiple partners who may either charge development time, support, or contract costs for simple changes.