Support modifying and exporting PKCS12

[rick_dean]

This patch adds support for modifying and exporting PKCS12.
The automated unit tests need some certificates from the
patch of https://bugs.launchpad.net/pyopenssl/+bug/348577
so this patch applies on top of that one.

[rick_dean (rick-fdd)]

This patch applies on top of the patch above. It
removes one debugging print that was accidentally
left in the patch. It also allows passphrase=None to
be treated like a missing passphrase or passphrase="".
There is a new test case for that too.

[rick_dean (rick-fdd)]

Apparently python 2.4 doesn't have Py_ssize_t which is the
return type for PySequence_GetItem() in 2.5. This patch
replaces that type to match int of previous releases
as I believe we are trying to support compatability back
to 2.3. Apply the patch over the one above.

I'm sure there is a better way to do this, such as
a preprocessor directive that is aware of the python
version, and defines Py_ssize_t when need be.

[rick_dean (rick-fdd)]

Oops. Py_ssize_t is the return type of PySequence_Length()
not PySequence_GetItem().

I wish this bug tracker numbered comments so I could indicate
which one I was referring to (#3 from the permalink).

[rick_dean (rick-fdd)]

Here is the better patch to replace comment 3. It
enhances src/pymemcompat.h to define the new
data type in python < 2.5.

[rick_dean (rick-fdd)]

I ported the changes to the tip of trunk, updated the documentation,
made PKCS12 a type just like most other objects, eliminated
Popen from the test cases, and fixed a warning. This branch is ready for merge.
lp:~rick-fdd/pyopenssl/pkcs12_mod_and_export2