Added support for Kelvin brightness temperature units in compute_flux.
Added a print statement warning about the Kelvin interpretation.
Wrote a test for compute_flux with kelvin inputs.
Updated documentation re: Kelvin brightness temperature units in flux.
Fixed a problem with the Kelvin flux computation; now we require & use spatial_scale.
Updated the Kelvin flux test case to reflect the new (correct) signature.
Updated the documentation for Kelvin and spatial_scale.
print "blah" -> print("blah") for python3 compatibility
Introduced UnitMetadataWarning to replace the unwieldy print() statement.
Fixed the omega_beam calculation; `wavelength` now uses spectral equivalency.

Addressing GitHub comments from Adam, Chris, Erik. Also massaged the warning for brightness temperature kelvins.
Fixed the first Kelvin test case, added a new one (with frequency input instead of wavelength)
Style cleanup with beams_per_pixel (includes omega_beam)

Also, the magic 1.1331 constant is gone, replaced with its more mathy fundamental expression of np.pi * 2 / (8*np.log(2))
Updated test cases to agree with the slightly more accurate beams_per_pixel.
Replaced commented radio-astro-tools link with a permament one.
Added support for Kelvin brightness temperature units in compute_flux.
Added a print statement warning about the Kelvin interpretation.
Wrote a test for compute_flux with kelvin inputs.
Updated documentation re: Kelvin brightness temperature units in flux.
Fixed a problem with the Kelvin flux computation; now we require & use spatial_scale.
Updated the Kelvin flux test case to reflect the new (correct) signature.