Add variadic zadd
Rename add_all to merge, add tests
add set command: srandmember support
noop if you try to add an empty array to a set

In my opinion, this should work:

```
foo = Redis::Set.new("foo")
foo << []
```

Currently it throws a Redis::CommandError

Also, added a test to make sure that adding nil to a set still adds an
empty element to the set
redis.rdb can no longer be an absolute path for some reason
Merge empty set fixes with redis-server test fixes
Merge pull request #117 from LYY/master

add set command: srandmember support
Merge branch 'variadiz-zadd' of https://github.com/hfwang/redis-objects
additional tests for @sorted_set.merge
Merge branch 'master' of github.com:nateware/redis-objects
Merge remote-tracking branch 'upstream/master'