Proof-of-concept `r.json` method using Oj gem

As discussed in #252, it sure is convenient to pass JSON strings directly to RethinkDB, or at least not have to instantiate all the objects just to convert them to RQL.

So I threw together a slightly different approach to the `r.json` method using the [Oj](http://www.ohler.com/oj/) JSON parser. Sorry, no tests because I can't get RethinkDB to build on my current machine. But some manual testing showed that the basic functionality works, at least.

Here is a quick benchmark using a 660KB JSON file ([code](https://gist.github.com/presidentbeef/724743fd7a11b57cfbf0#file-r_json_bm-rb)):

```
Rehearsal -------------------------------------------------------
JSON.parse + r.expr   8.600000   0.030000   8.630000 (  8.635941)
Oj.load + r.expr      8.440000   0.010000   8.450000 (  8.440146)
r.json                0.620000   0.000000   0.620000 (  0.617726)
r.json stream         0.560000   0.000000   0.560000 (  0.562764)
--------------------------------------------- total: 18.260000sec

                          user     system      total        real
JSON.parse + r.expr   7.950000   0.000000   7.950000 (  7.955163)
Oj.load + r.expr      7.960000   0.000000   7.960000 (  7.969099)
r.json                0.540000   0.000000   0.540000 (  0.538718)
r.json stream         0.540000   0.000000   0.540000 (  0.537156)
```

My conclusion: it's the conversion to RQL that is slow, not the JSON parsing.

I was a little surprised that streaming the file didn't perform better, but it may be that Oj is not actually streaming the data efficiently like I thought it would.