BUG: Allow projects to specify their own list of ITK COMPONENTS. #35

Setting ITK_FOUND to FALSE after calling find_package(ITK COMPONENTS
${components}) will cause CMake to re-run the find_package process with
a new set of COMPONENTS.
Merge pull request #36 from thewtex/find-itk-notfound

BUG: Allow projects to specify their own list of ITK COMPONENTS. #35