treebuilder: decode escape sequences in the 'append' command

Note that this might not work in python3 (we might need to encode the
data to utf8 before we can decode the escape sequences).

src/pylorax/treebuilder.py

@@ -288,7 +288,7 @@ class TemplateRunner(object):
 
     def append(self, filename, data):
         with open(self._out(filename), "a") as fobj:
-            fobj.write(data+"\n")
+            fobj.write(data.decode('string_escape')+"\n")
 
     def treeinfo(self, section, key, *valuetoks):
         if section not in self.results.treeinfo: