Finish reorganization

Now that @devinus has done the hard work,
there's still some things to do, namely:

- [ ] Correct a typo at `README`.
- [ ] Prefix **enums** as well. Currently they start with
  `MKD_` or `HTML_`.
- [ ] Review the README. Especially clarify the "bindings" part:
  all those bindings currently aren't Hoedown bindings.
- [ ] Rewrite the "Install" section. The "it's just three files" part
  is not true anymore.
- [ ] `Makefile.win` should be modified as well.
- [ ] Does `README`'s License match with `LICENSE`?
- [ ] Add **everything** to `hoedown.def`.