Fix docstrings, pep8, numpy use.

* Allow functions to work on non-arrays.
* Fix function docstrings so that they compile cleanly in sphinx.
* Make some (not all) PEP8-related changes.
* Small fixes for numpy use, such as np.any() instead of any().
move utils.reddening module to extinction
put back an accidentally removed import
correct scalar return value in extinction_fm
Unify reddening_* functions; add extinction(); docs

This commit does a few things:

* Unify reddening_fm() and reddening_ccm() functions into a single reddening()
  function with a keyword argument for different models. Currently there are
  five different models.
* Add an extinction() function that is the same is reddening, but in magnitude
  space. This is useful for testing and plotting because the original
  extinction laws are given in magnitude space.
* Expand and reorganize docs for the extinction() function, including
  plots comparing the extinction functions.
improve plot in extinction docs
Add basic extinction test for input, output shapes
Merge pull request #33 from kbarbary/extinction

Tidy up reddening function docs