plothist2d

Similar to `plothist`, I also implemented `plothist2d`. Simple function but fun to play with. Produces cool figures:
```
x=randn(10000)
y=randn(10000)

hd=[x y]
plothist2d(hd,[-3.0:0.1:3.0],[-3.0:0.1:3.0])
#plothist2d(hd,50,50)

file("fig2dim.png")
```

![fig2dim](https://f.cloud.github.com/assets/3612029/1498920/646d8880-4848-11e3-93c3-7b20f248c7d7.png)