Fix #5: Be opinionated and show namespace URIs rather than prefixes

tests/tree-output/basic-default-ns

@@ -1,6 +1,6 @@
 SimpleXML object (1 item)
 [0] // <movies>
-	->children('', true)
+	->children('https://github.com/IMSoP/simplexml_debug')
 		->movie[0]
 			->title[0]
 			->characters[0]

tests/tree-output/basic-ns

@@ -1,6 +1,6 @@
 SimpleXML object (1 item)
 [0] // <movies>
-	->children('test', true)
+	->children('https://github.com/IMSoP/simplexml_debug')
 		->movie[0]
 			->title[0]
 			->characters[0]
@@ -14,8 +14,8 @@ SimpleXML object (1 item)
 			->great-lines[0]
 				->line[0]
 			->rating[0]
-				->attributes('', true)
+				->attributes(null)
 					->type
 			->rating[1]
-				->attributes('', true)
+				->attributes(null)
 					->type

tests/tree-output/soap

@@ -1,15 +1,15 @@
 SimpleXML object (1 item)
 [0] // <soap:Envelope>
-	->children('soap', true)
+	->children('http://schemas.xmlsoap.org/soap/envelope/')
 		->Header[0]
-			->children('s', true)
+			->children('http://example.org/security')
 				->credentials[0]
-					->children('', true)
+					->children(null)
 						->username[0]
 						->password[0]
 		->Body[0]
-			->children('tns', true)
+			->children('http://example.org/some-soap-service')
 				->DoSomething[0]
-					->children('', true)
+					->children(null)
 						->from[0]
 						->to[0]