scale_x_date and scale_y_continuous improvements

added scale_x_date

added the ability to auto format labels in both
* scale_x_date
* and scale_y_continuous


for example you can do 


```python
ggplot(aes(x='date', y='beef'), data=meat) + geom_line() + \
    scale_x_date('date', labels="%Y-%m-%d") + \
    scale_y_continuous('beef', labels='comma')
```

this kind of stuff to format the date and put the y axis in a comma sep number format.