geom_bar expanded

Attempting to recreate more of the [ggplot2 geom_bar()](http://docs.ggplot2.org/0.9.3.1/geom_bar.html) functionality

Multicolor bar graphs expanded:
`ggplot(mtcars,aes(x='cyl',fill='factor(cyl)')) + geom_bar()`

![bar_factor](https://f.cloud.github.com/assets/2342749/2305544/cdfef9ec-a25b-11e3-956d-0c96afb79ecb.png)

Stacked bar graphs:
`ggplot(diamonds, aes(x='clarity',fill='cut')) + geom_bar()`

![stacked_bar](https://f.cloud.github.com/assets/2342749/2305547/149d2e50-a25c-11e3-9656-f25e42b91852.png)

Side by side bar graphs:
`ggplot(diamonds, aes(x='clarity',fill='cut')) + geom_bar(position='dodge')`

![dodge_bar](https://f.cloud.github.com/assets/2342749/2305546/02a92992-a25c-11e3-83ed-5c429f3c365b.png)

Also, added 'count' to ylab by default:

`ggplot(diamonds,aes(x='color')) + geom_bar()`

![bar_count](https://f.cloud.github.com/assets/2342749/2305549/4f5f7a52-a25c-11e3-94ff-94cc54d3e46d.png)

__ISSUES__:

How do I make the legend appear correctly? The code all seems to be part of `ggplot.py`. Right now I'm hacking by returning the callback `{"function": "legend","args":[]}`, but there must be a better way of doing this.

Is there a better way to get a color cycle? Right now I'm hard coding the thing.