`NoStore.Exists()`: Assume all device IDs exist

`NoStore.Exists()` now returns `true` instead of `false` for all device IDs—though this PR should be compatible with the `UAIDExists` flag that @jrconlin added in #120, since `testExistsHooks` lets us override specific device IDs.

Tests pass for:

* No storage.
* Fresh memcached server.
* memcached server containing existing entries.

@jrconlin, @bbangert r?
This uses the pure-Go memcached library; we could have Travis run this against a [hosted memcached](http://docs.travis-ci.com/user/database-setup/#Memcached), too.