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)

I should have commented it. From recollection, it is so that manual assignments to `fill` go through. e.g `geom_point(fill=['#00000', '#abcd00', ...])`. It may have been necessary for some tests as the `fill` aesthetic mapping hadn't yet been implemented.

However, that kind of specification is meant for the manual scales and I'm in favour of getting rid of any workarounds.