renamed for consistency; added stub.

--HG--
branch : pure-cffi
rename : pyrana/pixfmt.py => pyrana/pixelfmt.py
add docstring and fixes.

--HG--
branch : pure-cffi
consistency fixes.

--HG--
branch : pure-cffi
pep8/pylinit/consistency fixes.

--HG--
branch : pure-cffi
pep8 fixes.

--HG--
branch : pure-cffi
better singleton (at least is pythonic)

better API (naming)
move the setup here, where it belongs (...more).

--HG--
branch : pure-cffi
pep8/consistency fixes.

--HG--
branch : pure-cffi
Merge ../pyrana