Expand spectral() and spectral_density() equivalencies

This work is a result of discussions with @eteq at Astropy Coordination Meeting 2013. The conversion functionality added is important to `pysynphot`.

Summary of changes:

* pllim/astropy@8602dbe is a result of squashing a couple of commits I wish to forget.
* Added wave number to `spectral()` equivalencies and related tests.
* Fixed a bug in `spectral()` that left out conversion from `u.J` back to `u.Hz`. Added a test for it.
* `spectral_density()` now takes `Quantity` and not unit and value separately.
* Added PHOTLAM and PHOTNU conversions in `spectral_density()` and related tests.
* Added physical types for PHOTLAM and PHOTNU, and related tests.
* Updated related documentations and some outdated examples.
* The rest are minor PEP8 changes and my Emacs being awesome in auto-deleting trailing whitespace.