changes to make ruby-saml specs pass

 - change method signature of verify to accept an options hash
 - add ability to specify cert verification time
 - pull in ID attribute hack from ruby-saml, probably needs further
   investigation and documentation

also some other tweaks

 - switch to ruby native base64 decode, because why not?
 - un-pending the memory leak test for verify_signature; currently at
   6MB for 1000 iterations, would like to shrink.
 - pull in XmlNode struct and wire it in a few places for easier debugging
 - adding a saml assertion fixture and test for it, which exercises the
   fixed cert verification time feature