Add support for "reverse" lookups

[thisandagain]

Re: GH-3

How about using the existing emojiAliases dictionary and constructing the reverse, something like:

+ (NSDictionary *)reverseEmojiAliases {
    static NSDictionary *_reverseEmojiAliases;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        NSMutableDictionary *map = [[NSMutableDictionary alloc] init];
        NSSet* uniqueEmoji = [NSSet setWithArray:[[self emojiAliases] allValues]];
        [uniqueEmoji enumerateObjectsUsingBlock:^(NSString *emoji, BOOL *stop) {
            NSString* code = [[[self emojiAliases] allKeysForObject:emoji] firstObject];
            map[emoji] = code;
        }];
        _reverseEmojiAliases = [[NSDictionary alloc] initWithDictionary:map];
    });
    return _reverseEmojiAliases;
}

[sibahota059]

How about this?

-(NSDictionary *)reverseEmojiAliases {
NSMutableDictionary *_revEmojiAliases = [NSMutableDictionary new];
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    for (NSString *key in [EMOJI_HASH allKeys]) {
        _revEmojiAliases[EMOJI_HASH[key]] = key;
    }
});
return _revEmojiAliases;
}