Migration to C++17 and externalization of UnitTest++

[fwcd]

This branch migrates most of the Boost-dependent code to C++17 and requires UnitTest++ as a system dependency instead of storing the binaries inside of Git. This allows for a cleaner repository structure.

Since Clang <5 does not support C++17, there is an alternative branch that still uses Boost and C++11, but is otherwise identical.