ENH: External_*.cmake projects

This follows the Slicer conventions and allows project building TubeTK
to know what it depends on more easily.
It also clarifies the code by separating each dependency in its own
file.
ENH: Add option to choose the modules build

Adding option the option TubeTK_BUILD_ALL_MODULES. This option allows the
user not to build any module of TubeTK (only the libraries are built).
Also, if TubeTK_BUILD_ALL_MODULE is OFF, for each module of TubeTK, the
option TubeTK_BUILD_* appears so the user can choose modules one by one.
ENH: Clean up CLIs and library exporting

This allows projects outside of TubeTK to easily build each of TubeTK's
application or module. It also improves linking against TubeTK's libraries.
ENH: Move TubeTKConfig.cmake to superbuild directory

Much like CTK, it is easier for other projects building TubeTK to look for
The TubeTK superbuild directory instead of its inner build.
UseTubeTK.cmake and TubeTKTargets.cmake have been moved along as well.
ENH: Delete FindTubeTK.cmake

Find*.cmake should only be written to handle project's specific
configurations issues. Since TubeTK doesn't have any, the regular
find_package should be able to do its job without a FindTubeTK.cmake.