WS-Fed: Add some more error checking, reuse some simpleSAMLphp code, simplify some code.

Note that this patch has some user-visible changes:
- The entity id of the IdP is must now match the Issuer in the response we get
  from the IdP.
- The 'cert' option in the IdP is replaced with the 'certificate' option. The
  default search path of the certificate is the 'cert' directory.
- The AssertionConsumerService.php handler has been renamed to prp.php.
- The 'realm' option in the sp-hosted metadata is removed. Now we use the
  entity id of the sp instead.


git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@635 44740490-163a-0410-bde0-09ae8108e29a