Fixing cloudformation time stamps when API returns milliseconds

Found that boto was causing exceptions today when creating stacks in the ap-southeast-2 region. Tracked it down to the time stamp being returned by the cloudformation API containing milliseconds, for example '2013-05-15T06:59:38.245Z'. I tried creating stacks in us-west-1 and us-west-2 but didn't see this behaviour. I have opened a case with amazon to help understand why this would be.

In the mean time, this patch handles the two time stamp formats correctly.