Updated first key to work with binaries and tuple-encoded keys.
Updated first key to work with binaries and tuple-encoded keys.

With the changes to support time series the trick of using
an empty binary for the first key sorted *after* the tuple,
so switched to using a bare atom instead.

```
(dev1@127.0.0.1)7> <<>> < {a,b,c}.
false
(dev1@127.0.0.1)8> undefined < {a,b,c}.
true
```

andytill/riak_kv