add: error message constants has been added.
change: cache.set method will verify the type of key or val or both arguments before calling store.set method.
change: cache.delete method will verify the type of key argument before calling store.delete method.
change: cache.get method will verify the type of key or defval or both arguments and a returned value type of store.get method.
change: no need to check the type of arguments except val argument.
change: InMem.new function returns instance of Cache class.
change: inmem_try.lua has been updated.
remove: lib/item.lua and test/cache_try.lua has been removed.