Modify duration argument in cache set() method to denote expiry date which can be specified either as lifetime in seconds or exact expiration DateTime
Create trait to simplify implementation of new expiry argument semantics
Implement DateTime interpretation of expiry argument in most cache drivers
Implement DateTime interpretation of expiry argument in file cache driver
Add namespace import and exception class
Merge pull request #8 from ob-ivan/feature/cacheExpireDatetime

Allow expiry date in cachers to be specified as a DateTime object