add rc param for centimeter support

It would be great if I could specify a figsize in centimeters. I would like to write

```
import matplotlib as mpl
mpl.rcParams['lengthunit'] = 'c'
figure(figsize=(12, 8))
```

instead of

```
figure(figsize=(12/2.54, 8/2.54))
```

I think this would be a great addition for the non-anglo-saxon world ;)
