Support timestamps without microseconds in Cloudwatch alarms.

Some of the older CloudWatch alarms are failing in the `strptime` call because the timestamp doesn't match the format given in boto:

```
  File "workspace/boto/boto/ec2/cloudwatch/__init__.py", line 448, in describe_alarm_history
    [('member', AlarmHistoryItem)])
  File "workspace/boto/boto/connection.py", line 1037, in get_list
    xml.sax.parseString(body, h)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/sax/__init__.py", line 49, in parseString
    parser.parse(inpsrc)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/sax/expatreader.py", line 107, in parse
    xmlreader.IncrementalParser.parse(self, source)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/sax/xmlreader.py", line 123, in parse
    self.feed(buffer)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/sax/expatreader.py", line 207, in feed
    self._parser.Parse(data, isFinal)
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/xml/sax/expatreader.py", line 304, in end_element
    self._cont_handler.endElement(name)
  File "workspace/boto/boto/handler.py", line 38, in endElement
    self.nodes[-1][1].endElement(name, self.current_text, self.connection)
  File "workspace/boto/boto/ec2/cloudwatch/alarm.py", line 313, in endElement
    '%Y-%m-%dT%H:%M:%S.%fZ')
  File "/System/Library/Frameworks/Python.framework/Versions/2.6/lib/python2.6/_strptime.py", line 325, in _strptime
    (data_string, format))
ValueError: time data '2013-01-14T11:28:03Z' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'
```

This fix will try the non-microsecond timestamp format if the initial format fails.