Bring `{Emcee, No}Store` in line with `GomemcStore`

`{Emcee, No}Store.Exists()` now correctly returns `false` if a device ID has not previously registered with the push server. This is technically backward-incompatible, but is the expected behavior—`Exists()` shouldn't return `true` if the device ID doesn't actually exist.

All tests now pass for the `memcache_gomc`, `memcache_memcachego`, and `none` storage adapters. \o/

@jrconlin r?