Use 'own' InvalidArgumentException to properly match constructor signature
Added TRANSFORMATION_ENVELOPED_SIGNATURE constant
Prevented notice: "Use of undefined constant 'self'"
Added support for enveloped signature canonicalization.
Added support to force an URI attribute on the <Reference/> element.
Re-use namespace constant and namespace-prefix constant
Improved support for empty URI attributes on the <Reference/> element
Merge pull request #13 from holtkamp/enveloped-signature-canonicalization

Enveloped signature canonicalization