Set non-required flag for texture filename attrib

Fix problems with loading URDF with empty texture tag from baxter's
dynamically loaded URDF on the param server.

src/urdf_parser_py/urdf.py

@@ -149,7 +149,7 @@ def __init__(self, filename = None):
 		self.filename = filename
 
 xmlr.reflect(Texture, params = [
-	xmlr.Attribute('filename', str)
+	xmlr.Attribute('filename', str, required=False)
 	])