Option to use Ox as a SAX handler

I've run [feedjira-benchmarks](https://github.com/feedjira/feedjira-benchmarks) on this and it definitely looks like Ox makes sax-machine more than 30% faster.

Here are the results:
```
Rehearsal -----------------------------------------------
feedjira     15.490000   0.070000  15.560000 ( 15.703976)
feedjira-ox  10.530000   0.030000  10.560000 ( 10.675326)
------------------------------------- total: 26.120000sec

                  user     system      total        real
feedjira     15.640000   0.060000  15.700000 ( 15.850888)
feedjira-ox  10.520000   0.030000  10.550000 ( 10.667229)

```

![others-benchmark](https://cloud.githubusercontent.com/assets/944286/3222275/6f08db22-f014-11e3-8f3a-63f1d97dd982.png)