opaque_pointer bug fix + enhancement

[ganssauge]

After debugging a crash I found out that all instances
created from opaque_pointer_converter had a type object
which itself didn't have a type, and this finally lead
to the crash.

The attached patch corrects this behaviour by
initialising the type object using PyType_Ready().

Furthermore I implemented cross module support in so
far as the converter registers itself only if not
another module already has registered the same
converter before.
The opaque test case has been update to check for the
segmentation fault mentioned in my previous post.
A test case for the cross module support is included
(crossmod_opaque).

I also updated the documentation for opaque.

I tested my patch with
 - GCC-4.1 running on GNU/Linux 2.6.16.21-0.13-default
x86_64
 - Visual-C++-6.5 running on Windows XP
 - Visual-C++-7.1 running on Windows XP

The patch is applied by calling
	patch -p1 < opaque_patch
in the root of the boost hierarchy.

Files patched are:
boost_head/boost/python/opaque_pointer_converter.hpp
boost_head/libs/python/doc/v2/opaque.html
boost_head/libs/python/doc/v2/opaque_pointer_converter.html
boost_head/libs/python/doc/v2/reference.html
boost_head/libs/python/doc/v2/return_opaque_pointer.html
boost_head.orig/libs/python/test/crossmod_opaque_a.cpp
boost_head/libs/python/test/crossmod_opaque_a.cpp
boost_head/libs/python/test/crossmod_opaque_b.cpp
boost_head/libs/python/test/crossmod_opaque.py
boost_head/libs/python/test/Jamfile
boost_head/libs/python/test/Jamfile.v2
boost_head/libs/python/test/opaque.py

[ganssauge]

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it has been applied to CVS Head