Revert "Colortable Image Source: Fixed a bug in the slow (non-vigra) render path."

This reverts commit 4b4a74f384218de633e032d7c971752fa257781f.
Revert "cast before doing vigra.colors.applyColortable"

This reverts commit 5e7c295919c3471852c61f9e2eb0c29ea6192923.
added skeletons to setup.py and fixed minimal example
optional dependency checks for vigra and vtk
added nosestes as a dependency to setup.py
more optinal vtk imports
made lazyflow import optional
better optional dependency checking for lazy_test.py
Update Readme.md
Update Readme.md