Add latex for R_sun and L_sun, switch order with solRad

R_sun and L_sun weren't rendering properly in iPython notebook because they didn't have latex representations:

    >>> u.M_sun._repr_latex_()
    u'$\\mathrm{M_{\\odot}}$'

    >>> u.R_sun._repr_latex_()
    u'$\\mathrm{solRad}$'

...so I fixed that! :dancer: