Adding few additional changes to the gem in order to make response succeed on SalesForce Saml Request Validator. Also including some additional parameters to  validate_saml_request method in order to include some additional custom values. Finally moving to "xmldsig" for message signature.

Squashed commit of the following:

commit 3726c2120a62d0a66e388eb1e8b1f765ac76133b
Author: Rodrigo Rodriguez <rrodriguez@growthaccelerationpartners.com>
Date:   Wed Apr 2 08:57:48 2014 -0600

    Adding a new option in the validate_saml_request method to include the xml_tag also fixing an error with digest url if using a non SHA1 algorithm.

commit b602bfc8468e405f658f3e16419c254c0b50580b
Author: Rodrigo Rodriguez <rrodriguez@growthaccelerationpartners.com>
Date:   Mon Mar 31 14:24:30 2014 -0600

    Adding few additional adjustments on the code in order to sign the information correctly and be sure that it works using SalesForce SAML validator.

commit abe6a73f2ca7a7d81a79df4f5dfb7faeb70e716d
Author: Rodrigo Rodriguez <rrodriguez@growthaccelerationpartners.com>
Date:   Thu Mar 27 10:59:59 2014 -0600

    Adding few additional changes on the XML message to pass Sourceforce validation.

commit f2b7b75bc86c1126a398f527f2947f4f70c5ff67
Author: Rodrigo Rodriguez <rrodriguez@growthaccelerationpartners.com>
Date:   Tue Mar 25 22:24:33 2014 -0600

    Adding a new XML format