Geom pointrange

Hi,

I implemented the geoms geom_linerange and geom_pointrange which are needed for stat_summary (which I already started to implement). I hope my implementation is okay, I will be happy to change everything that you wish.

At the moment, geom_linerange and geom_pointrange are implemented independent. That is not as elegant as it could be, as geom_pointrange is essentially just geom_point+geom_line. However, I am not sure how I could implement it this way. Any advice would be appreciated.

### Example

![test](https://cloud.githubusercontent.com/assets/4093707/3512173/b96855ea-06b6-11e4-83f0-1a381813ff55.png)

To make the tests pass, I had to remove all tests of fill mappings in https://github.com/yhat/ggplot/blob/master/ggplot/tests/test_colors.py#L82 (i.e. from this line to the end of `test_assign_colors`. Is this okay? Or how do I have do adapt them to be working again?