Parallel CountVectorizer

![figure_1_mem](https://f.cloud.github.com/assets/786402/316331/6dabd876-9811-11e2-9f4e-21347b19c829.png)
![figure_1_transform](https://f.cloud.github.com/assets/786402/316332/6db4c54e-9811-11e2-926f-362e84ac8e34.png)
![figure_1](https://f.cloud.github.com/assets/786402/316333/6dbd6514-9811-11e2-9182-f703a5976f3b.png)
Slightly higher memory usage for multiprocessing but much faster.