Moved ValueHolder implementation out of OnHeapStore into new type and added unit test for it.
Merge pull request #95 from vfunshteyn/issue-46

Issue 46
Issue #46 minor cosmetic change
Issue #31 Added compute* on Store
Issue #31 Added stub CacheWriter(Factory) interfaces and wired in Ehcache
Issue #52 Somewhat resilient Cache.get() using Loaders
Issue #31 introduced method to deal with null producing Function and BiFunction
Issue #52 Introduced Memoized function call to load