Default ui should confirm everything for easy usage of pisi (without a need to create own subclass of ui)

> import pisi
> pisi.api.install(["amsn"])

or else this returns False.

pisi/ui.py

@@ -72,7 +72,8 @@ class UI(object):
 
     def confirm(self, msg):
         "ask a yes/no question"
-        pass
+        # default ui confirms everything
+        return True
 
     def display_progress(self, **ka):
         "display progress"