Bug fixes for spectral_cube.py

Fixes bug mentioned in issue #178, and deals with the Quantity/ImageHDU issue.

----
from astropy.units import Quantity
energy_range = Quantity([10, 500], 'GeV')
flux_background = fermi_diffuse.integral_flux_image(energy_range)
print flux_background.data
[[  4.96538718e-08   5.08790160e-08   5.16831888e-08 ...,   8.34615912e-08
    8.27155529e-08   8.25761185e-08]
 [  5.15609632e-08   5.20233728e-08   5.20025838e-08 ...,   8.38200572e-08
    8.27546690e-08   8.26082274e-08]
 [  5.66285891e-08   5.53183727e-08   5.51376924e-08 ...,   8.54037376e-08
    8.49908847e-08   8.45082814e-08]
 ..., 
 [  7.91854550e-08   7.92956875e-08   8.16965935e-08 ...,   7.36334291e-08
    7.59015607e-08   7.62683800e-08]
 [  7.86396735e-08   7.92222013e-08   8.11234297e-08 ...,   7.34012929e-08
    7.62491752e-08   7.74276834e-08]
 [  7.84162988e-08   7.85808745e-08   8.04845338e-08 ...,   7.27042248e-08
    7.62317234e-08   7.81416615e-08]]

energy_range = Quantity([10E3, 500E3], 'MeV')
flux_background = fermi_diffuse.integral_flux_image(energy_range)
flux_background.data

array([[  4.96538718e-08,   5.08790160e-08,   5.16831888e-08, ...,
          8.34615912e-08,   8.27155529e-08,   8.25761185e-08],
       [  5.15609632e-08,   5.20233728e-08,   5.20025838e-08, ...,
          8.38200572e-08,   8.27546690e-08,   8.26082274e-08],
       [  5.66285891e-08,   5.53183727e-08,   5.51376924e-08, ...,
          8.54037376e-08,   8.49908847e-08,   8.45082814e-08],
       ..., 
       [  7.91854550e-08,   7.92956875e-08,   8.16965935e-08, ...,
          7.36334291e-08,   7.59015607e-08,   7.62683800e-08],
       [  7.86396735e-08,   7.92222013e-08,   8.11234297e-08, ...,
          7.34012929e-08,   7.62491752e-08,   7.74276834e-08],
       [  7.84162988e-08,   7.85808745e-08,   8.04845338e-08, ...,
          7.27042248e-08,   7.62317234e-08,   7.81416615e-08]])


----
So these are now the same (sorry, I haven't figured out how to write code blocks on here).