Refactor code that generates scikit-image logo.

Simplify code and use skimage functions that didn't exist when this code was
originally written.
STY: PEP8 for logo.
Merge pull request #4 from stefanv/tonysyu_logo_refactor

STY: PEP8 for logo.
Merge pull request #457 from tonysyu/logo_refactor

RF: Refactor code that generates scikit-image logo.