multi bar plots added

Multibar graphs added

NOT READY TO PULL
------

Still need to add docs and tests, but wanted to share this for code review.



```
from ggplot import *
ggplot(diamonds,aes("clarity",fill="cut")) + geom_bar()
```

![multibar](https://cloud.githubusercontent.com/assets/2342749/3251965/800f0ef4-f1b9-11e3-8d0a-1c2190b532ce.png)

```
from ggplot import *
ggplot(diamonds,aes("clarity",fill="cut")) + geom_bar(position="dodge")
```

![multibar_dodge](https://cloud.githubusercontent.com/assets/2342749/3251970/988be0ce-f1b9-11e3-84b6-15f32bdf38d4.png)