fix build.

Don't put hard path to find the libsodium library. Instead, rely on
pkg-config.
add Makefile

remove shell scripts and just use a makefile, while I'm here move the
test in src folder to not pollute the root directory.
let github handle the readme markdown
remove txt extension from the license file.

The extension is unusal and useless.
Merge pull request #1 from refuge/master

some simple changes to the build