Ninja (replacement for Makefile)

This branch is **not for merging** (yet). But I'm opening a pull request, because I'd like to get some feedback on it. Can people test this, report issues, and propose improvements?
* run `./configure` (`--help` for a list of options) to generate a build.ninja
* run `ninja` to build (`sudo apt-get install ninja-build` to install on Debian based distros)
Thanks!