Initial support for AIX big archive format.


Problem description
===================

This branch add support for AIX AR big format archive as documented here: https://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.files/doc/aixfiles/ar_big.htm


Changes description
===================

A new archive class was created since the format is different than BSD/GNU.

file type is reported as HEADER_GENERIC

file header for each file from archive is variable and this is why it is parsed from 2 read operations.

I have switched some methods from hidden (__X) to private (_X) so that I can reuse them in the new code.

New code is written with PEP8 and PEP257 (for docstring) styleguide...with the exception of tab indentation.
Please let me know if I should use other styleguide.

The code is written using TDD and this is why all tests are placed in a single file. There is no dedicated test file for failures. All tests for the new format are located in a single file, with docstring documenting the purpose of each tests. Also, no external / system files were used for testing, all file content is in memory or generated in tests.


How to try and test the changes
===============================

Please check that changes make sense and let me know if anything needs changes.

If you want to check a real file here are some files copied from an AIX system:

Simple archive libg.a
https://dl.dropboxusercontent.com/u/174543/aix-ar/libg.a

libpam with 2 members, with initial padding and 1 with padded header  and anotehr without padded header
https://dl.dropboxusercontent.com/u/174543/aix-ar/libpam.a

shr.0 from libpam.a extracted using aix ar tool:
https://dl.dropboxusercontent.com/u/174543/aix-ar/shr.o

Thanks!