Fix extinction units

This is my first pull-request, so I hope I'm doing this right! I faced some problems when trying to use the extinction laws included in specutils. Specifically, the code as written introduced extraneous units to the extinction laws, returning astropy quantities with units of Angstroms instead of the expected dimensionless quantities.

These changes aim to fix that problem. I have removed code which introduces the wavelength units to the extinction, and changed certain aspects of the docstrings which indicated functionality not yet implemented (specifically, the freedom to specify either A(V) or E(B-V): the extinction functions as written expect to be passed A(V), and the functinality to check the inputs and convert E(B-V) to A(V) doesn't seem like it's been integrated into the function calls, though I might be wrong about that.)

I have not figured out how to run the test suite yet, so I don't know if I've broken something subtle, but I have been able to import and use the reddening() function in my research code and got expected results - something I could not do before my changes (since the exponent was not unitless before these changes.)