bugfix: makefile not install external/ headers into subdir
add sha256 hash test case
add proposed functions.
add some comments.
add alg detail to comments for hash.hpp
rename hash functions to terser forms.
added ripemd hash unit test
remove comment (I've verified sha256 stuff works)
move ripemd test into its own thing
bugfix: ripemd was being passed wrong size. now generates correct hashes.