[Review]: Functions to calculate GOES radiative losses

Hi guys,
    Here is the next batch of software for analysing GOES data in sunpy.  This PR includes functions to calculate the radiative losses from the GOES-observed plasma, calc_rad_loss(), and the X-ray luminosity in the wavelength ranges of the two GOES channels, goes_lx().  There are also two functions which allows these properties to be calculated and inserted directly into and GOESLightCurve object, rad_loss_rate() and xray_luminosity, respectively.

Let me know what you guys think.  For instance, there is probably a better place than goes.py to put _time_steps(), which calculates the time differences between successive measurement times.  I'm also not sure if the assert statements in my units tests are the most efficient.  

However, all the function give the correct results.  If the functionality proposed here can be merged, sunpy could allow you to do all the analysis of GOES data that is possible in SSW which I think would be a great step forward.
I would say that this should be `radative_loos_rate` I feel the 5 extra characters are worth the clarity ;)

Also I would rename `download` to `force_download` because it will overwrite the file, correct?