Require the PSR/Cache package.
Update for multi-set operations.
Update for constants on saving.
Split deferring logic off to a trait.
Update setExpiration() for interface.
Fix bug with misnamed variable and bad int checking.
Switch to a separate saveDeferred() method.
Minor tweak to the test file.
Another tweak to the test script.
Merge pull request #1 from Crell/multi-set

Multi set support