issue #72 - cache expiration
Merge pull request #140 from timeck/issue-72

issue #72 - cache expiration
more key/value type checks
added test for new key/value type checks. filled out SPI tests for
Store.computeXXX() methods
review comments:
- remove junit method calls in SPI test
- Allow CacheAccessException to occur w/o failing test
some more review comments