Explicitly autorelease realms returned from RLMRealm class methods

Currently whether they are or aren't depends on the whims of ARC, which
means that it's inconsistent as to whether or not the following code
works:

    [[RLMRealm defaultRealm] beginWriteTransaction];
    // ...
    [[RLMRealm defaultRealm] commitWriteTransaction];

Currently this works only in debug modes or if there is anything else
keeping the realm alive, which is rather confusing.