support.

The TarInfo class now contains all necessary logic to process and
create tar header data which has been moved there from the TarFile
class. The fromtarfile() method was added. The new path and linkpath
properties are aliases for the name and linkname attributes in
correspondence to the pax naming scheme.

The TarFile constructor and classmethods now accept a number of
keyword arguments which could only be set as attributes before (e.g.
dereference, ignore_zeros). The encoding and pax_headers arguments
were added for pax support. There is a new tarinfo keyword argument
that allows using subclassed TarInfo objects in TarFile.

The boolean TarFile.posix attribute is deprecated, because now three
tar formats are supported. Instead, the desired format for writing is
specified using the constants USTAR_FORMAT, GNU_FORMAT and PAX_FORMAT
as the format keyword argument. This change affects TarInfo.tobuf()
as well.

The test suite has been heavily reorganized and partially rewritten.
A new testtar.tar was added that contains sample data in many formats
from 4 different tar programs.

Some bugs and quirks that also have been fixed:
Directory names do no longer have a trailing slash in TarInfo.name or
TarFile.getnames().
Adding the same file twice does not create a hardlink file member.
The TarFile constructor does no longer need a name argument.
The TarFile._mode attribute was renamed to mode and contains either
'r', 'w' or 'a'.