Finish reorganization

Now that @devinus has done the base work,
there's still some things to do, namely:

#### Code (3)

- [x] Prefix **enums** as well. Currently they start with
  `MKD_` or `HTML_`.
- [x] Normalize guard names and comments on headers.
- [x] General cleanup.


#### Building and versioning (4)

- [x] Remove the `html/` directory from Makefiles.
- [x] `Makefile.win` should be modified as well.
- [x] Add **everything** to `hoedown.def`.
- [x] Reset version.


#### Readme and licensing (5)
- [x] Correct a typo at `README`.
- [x] Review the README. Especially clarify the "bindings" part:
  all those bindings currently aren't Hoedown bindings.
- [x] Rewrite the "Install" section. The "it's just three files" part
  is not true anymore.
- [x] Does `README`'s License match with `LICENSE`?
- [x] Update preambles where necessary.