Make CMAKE_BUILD_TYPE a cache variable so it can be changed at
build type if using ccmake or cmake-gui or after initial configuration
using `make edit_cache`.
Provide configurable option SANITIZE so user can easily tweak it.
Forcing the C++ compiler is now done in the cache so the user
can tweak the location of clang.
Rewrote install guide for makefiles noting the use of ccmake and cmake-gui
Merge pull request #56 from delcypher/cmake_features

Cmake features