Convert yml2 into a Python package.

* Rename yml2.py -> yml2/__init__.py
* Rename backend.py -> yml2/backend.py
* Rename pyPEG.py -> yml2/pyPEG.py
* Use relative imports where appropriate.
* Change imports in yml2c, yml2proc.
* Update make target `clean`.

include homepage.en.yhtml2

page "Hello, world" {
    p   >>
        Hello, world! I can link here, say:
        ¬http://en.wikipedia.org to Wikipedia¬
        >>

    p   >>
        This is ƒemphasized. And this is «code».
        >>
}