Cosmetics in setup.py.
Proper match/target size computation.

Use xt_align() when computing the size of a target/match backing buffer.
Comment cosmetics.
Store pointer to backend buffer.

In a target or match object self._module.t should point to the backing
buffer (self._ptr). The new API x6_parse() function and extensions expect
this.
Clean up indentation: 4 spaces only.
Use new API.

The new API uses optional new callback functions in extensions. Now
we are also compatible with extensions using x6_parse().

TODO: move stuff from xtables.py to __init__.py, and override parse
in the Target or the Match class, calling x6_parse() from there.