Example app fails on 2nd & subsequent requests

The example app, on first request, writes a session cookie. On subsequent requests,
it tries to read that cookie in and convert 'last_request_at' to an isoformatted date
string. However, 'last_request_at' is an integer epoch date, and ints have no isoformat()
method. I chose to change the cookie-reading code rather than the writing code here.
Merge pull request #6 from bkjones/example_app_fix_session_timestamp

Example app fails on 2nd & subsequent requests