getCreatedOn and getExpiration methods and tests

This adds two methods which reveal a record's expiration and createdOn timestamps, if they exist. I also added two tests to the AbstractCacheTest file, though they're fairly simple. Based on existing methods, both of these return `null` if the record doesn't exist or a timestamp can't be returned. 

All local non-skipped phpunit tests are passing (45 skipped, no redis, etc). All tests in AbstractCacheTest pass. [Travis passes](https://travis-ci.org/joemaller/Stash/builds/18599179) too.

Thanks to @Ninestein in #68