Add Draine extinction curves

I have added the Draine extinction curves, both Weingartner & Draine (2001) and Draine (2003). These extinction curves are given as tables generated from a dust model and cover a very broad range of wavelengths (from 1 angstrom to 1 millimeter). Currently the curves are limited to r_v = 3.1, 4.0 and 5.5. A possible future improvement would be to provide interpolation between these values.

Here is the comparison with the currently-implemented extinction laws. 

![extinctioncomparison](https://f.cloud.github.com/assets/3639698/1585919/ee206786-5221-11e3-8a01-297b4eba6ae5.png)

I have tried to follow the existing format for documentation and other things.