Use proper base timestamp when setting cookie TTL

The PiwikTracker PHP class was using the createTs as a base
for applying the cookie TTL offset instead of using the currentTs.

This will result in cookie TTLs being in the past TTL seconds
after the initial visitorId cookie is created. For example, 30
minutes after the initial visitorId cookie is set, all "ses"
cookies will start getting set with expiration times in the past.
Merge pull request #246 from brownoxford/master

Use proper base timestamp when setting cookie TTL