Support for metadata with leading XML comment

[tmilos]

Copy of aerialship/lightsaml#34

@relaxnow reported

Trying to load metadata from: https://engine.surfconext.nl/authentication/idp/metadata yields the following error:

Catchable Fatal Error: Argument 1 passed to 
AerialShip\LightSaml\Model\Metadata\EntityDescriptor::loadFromXml() 
must be an instance of DOMElement, instance of DOMComment given, called in 
/vagrant/vendor/aerialship/saml-sp-bundle/src/AerialShip/SamlSPBundle/Config/EntityDescriptorFileProvider.php 
on line 91 and defined in 
/vagrant/vendor/aerialship/lightsaml/src/AerialShip/LightSaml/Model/Metadata/EntityDescriptor.php 
line 156

It appears to fail at the leading XML comment which is required by eduGain.