Correct bug in calculation of sunset without PyEphem.  Also document the source of the formulas.
Correct bug in calculation of sunset without PyEphem.  Also document the source of the formulas.

git-svn-id: acf0ef94bfce630b1a882387fc03ab8593ec6522@218 150532fb-1d5b-0410-a8ab-efec50f980d4

ahaberlie/MetPy,Unidata/MetPy,jrleeman/MetPy,ahill818/MetPy,dopplershift/MetPy,ShawnMurd/MetPy,dopplershift/MetPy,Unidata/MetPy,jrleeman/MetPy,deeplycloudy/MetPy,ahaberlie/MetPy