Docstrings for modules all over sugar and shell.

These are just the doc strings I created as I was spelunking
through to see how Sugar manages launching applications.  The
resulting auto-documentation is neither polished or finished,
but it should help people reading the code somewhat.

There are a few minor code cleanups:

  * activityhandle (replacing C idiom for initialisation with
    a Python one)
  * bundle registry (using a parameterised directory name so
    that it shows up in the documentation)
  * validate_activity_id function, use isinstance( item, (str,unicode))
    for the query, rather than two separate checks with isinstance

sugar/date.py

@@ -1,10 +1,25 @@
+"""Simple date-representation model"""
 import datetime
 
 class Date(object):
+    """Date-object storing a simple time.time() float
+    
+    XXX not sure about the rationale for this class,
+    possibly it makes transfer over dbus easier?
+    """
     def __init__(self, timestamp):
+        """Initialise via a timestamp (floating point value)"""
         self._timestamp = timestamp
 
     def __str__(self):
+        """Produce a formatted date representation
+        
+        Eventually this should produce a localised version 
+        of the date.  At the moment it always produces English
+        dates in long form with Today and Yesterday 
+        special-cased and dates from this year not presenting
+        the year in the date.
+        """
         date = datetime.date.fromtimestamp(self._timestamp)
         today = datetime.date.today()