Check that keys are strings
Bug: there is no "remove"-command in the memcache module, changing to "del".
Implemented delMulti. There is no bulk delete command in memcache, but this will run one series of delete commands in parallell per memcache host.
Implemented setMulti and updated tests to cover the new multi-functions.
Merge pull request #2 from bloglovin/feature/more-multi

Feature/more multi