Add wavelength uncertainty from SFD98

Add the possibility of wavelength uncertainty in the table from SFD98.
Still does not bring the extinction estimate to within tolerance.
Restored disagnostic figure.
Documentation says that Fitzpatrick 1999 model is default

Updated the inline documentation to note that Fitzpatrick 1999 model is
the default, as this is otherwise not visible to the user.
Added documentation for Draine dust models
Added wd01 model

wd01 model is only defined at r_v = 3.1, 4.0 and 5.5, although we
interpolate to all wavelengths. Added the datafiles describing this
model. Currently normalize extinction to be 1 at 5495 angstroms, in
keeping with CCM89.
Finish wd01 model

Added datafiles and code to implement the wd01 model (in addition to
d03). These extinction laws have small but significant differences.
Correct path to datafiles. Add wd0/d03 to compare plot
Remove test plot
Fix test_extinction and model list

Fix my incorrect old/test version of test_extinction. Update list of
extinction models.
Put Draine model data in sub-directory

Update setup.py to find sub-directory
Mark default extinction with proper ReST
cache splines in Draine dust models