#117 fix SPI tests by adding missing key/value type checks
 #117 move SPI test methods so we're DRY-ready once other Store implementations are on the way
 #117 fix wrongValueTypeThrowsException to make it check the value type and not the key type
Merge pull request #123 from lorban/issue-117

 #117 fix SPI tests by adding missing key/value type checks
Issue #75: wired CacheLoader where applicable.
Issue #75: corrected method signatures to include CacheLoader/CacheWriter exceptions where applicable.
Issue #75: split fix for OnHeapStore.bulkComputeIfAbsent() from main commit.
Merge pull request #121 from vfunshteyn/issue-75

Issue #75: wired CacheLoader where applicable.
draft of Expiration and impl in OnHeapStore