AuthnRequest customization

[simevo]

users of this package should be able to customize following fields:

• AssertionConsumerServiceURL
• AssertionConsumerServiceIndex
• AttributeConsumingServiceIndex
• AuthnContextClassRef

[simevo]

related to #18

the Italia\Spid\Sp::login function interface should be:

// performs login
// $idpName: shortname of IdP, same as the name of corresponding IdP metadata file, without .xml
// $level: SPID level (1, 2 or 3)
// $redirectTo: return url
// $ass: index of assertion consumer service as per the SP metadata
// $attr: index of attribute consuming service as per the SP metadata
public function login($idpName, $redirectTo = '', $level = 1, $ass = 0, $attr = 0);