STYLE: Convert CMake-language commands to lower case

Ancient CMake versions required upper-case commands.  Later
command names became case-insensitive.  Now the preferred style
is lower-case.
This also changes all the key words to lower case. The
primary reason for changing key words is that all
documentation for CMakeLists.txt now shows the key words as
lower case. Even the printed “Mastering CMake v5” uses lower
case.
ENH: Rename txx to hxx for ITKv4 conventions.
STYLE: Use .hxx instead of .txx to match ITKv4 style.
STYLE: Format in ITKv4 style.
Merge pull request #1 from hjmjohnson/UpdateStyleToITKv4

Update style to it kv4