Fix opening the output file in either normal or binary mode

Fix the etree in python lxml wanting to have the file opened in the 'wb'
mode. Fixes the 'TypeError: write() argument must be str, not bytes'
error message but retains compatibility if lxml would be missing. Closes
issues tjfontaine#26 and tjfontaine#28

airprint-generate.py

@@ -220,7 +220,7 @@ def generate(self):
                 if self.directory:
                     fname = os.path.join(self.directory, fname)
 
-                f = open(fname, 'w')
+                f = open(fname, 'wb' if etree else 'w')
 
                 if etree:
                     tree.write(f, pretty_print=True, xml_declaration=True, encoding="UTF-8")