Limits

I've added partial axis limits to xlim and ylim geoms
    
if 'g' is a plot, then we can specify partial limits as:
- g + xlim(0) # For a lower x limit of 0
- g + ylim(high = 4) # for a higher y limit of 0
- g + xlim(None, 42) # for a higher x limit of 42
- g + ylim(low = 2) # for a lower y limit of 2
    
The unspecified limit is set as it would have been in a plot without explicit limits added. I.e if 'g' is plotted without setting xlimits and the plot range from 0 to 6, then plotting 'g + xlim(low = 4)' would adjust the xlimits to 4 to 6

I've included a test case covering this addition.

I've tried to follow the git part of the [contribute guide](https://github.com/yhat/ggplot/blob/master/docs/contributing.rst) to the best of my abilities, but if I've fallen short on some aspects, just let me know and I'll do my best to modify things.

Jonas