radon: Use util.pad for array padding.
iradon: Use util.pad for array padding.

This depends on util.pad accepting 0s in pad_width, which has a proposed
solution in gh-634.
iradon: Reduce code duplication.
radon: Use numpy.interp/scipy.interpolate.

Scipy supports all interpolation kinds (nearest, linear) we need, while
numpy supports only linear interpolation. The numpy interpolation is
substantially faster, so this is used even though it complicates the
code slightly.
iradon: Only allow interpolation methods working well.

Of the interpolation methods provided by scipy.interpolate.interp1d,
only cubic has been found to work well with the tests in skimage (other
methods are either identical to linear or nearest, or they produce bad
reconstructions).
iradon: Add test for cubic interpolation.
iradon: Cleanup by locating related code in one place.
iradon: use util.pad for sinogram padding.
Merge pull request #635 from josteinbf/radon-cleanup

Clean up radon and iradon to improve readability