Add support for signature verification

In working on expanding our documentation for our OIDC deployment at MIT, I noticed that none of the PHP libraries out there seem to verify signatures.  This adds preliminary support for verifying signatures (RSA only), using [phpseclib](http://phpseclib.sourceforge.net/).

It also ensures that the base64urlencoded data is decoded correctly, by implementing base64 url decoding as described in RFC4648 section 5, "Base 64 Encoding with URL and Filename Safe Alphabet"