Fix comment typo.
Add doc/_build to .gitignore.
Fix IPv6 prefix length parsing.

When specifying an IPv6 address, accept normal numeric prefix lengths.
Update IPv6 class comments.
Don't use global constants.

Don't create tables and policies as constants in ip4tc and ip6tc,
since this prevents to import iptc if we don't have root privileges.
This makes it easier to e.g. generate documentation without using
sudo.

This changes the calling semantics somewhat, e.g. to get access to
the filter table use:

    >>> from iptc import Table
    >>> table = Table(Table.FILTER)
Fix Rule6?._get_tables().
Update Table creation in tests.
Update Table docstrings.
Update Table6 docstrings.
Update documentation.
No need to sudo now in Makefile.