add release notes for 1.51 (unpacking patterns) and clean up example in unpacking patterns section

[SVN r79703]
accomodate fusion:vector0 interface change

[SVN r53919]
add non-template copy-assign operator

[SVN r66368]
Remove obsolete MSVC check from pragma guard

 git grep -h -B1 "^#\s*pragma once" | grep -v pragma | sort | uniq

is now clean.

[SVN r85952]
pre-preprocess the contexts

[SVN r71841]

Conflicts:
	include/boost/proto/context/default.hpp
	include/boost/proto/context/detail/default_eval.hpp
	include/boost/proto/context/detail/preprocessed/default_eval.hpp
	preprocess/preprocess_proto.cpp
finish pre-preprocessing proto/transform and proto/detail. only proto/context remains

[SVN r71832]

Conflicts:
	include/boost/proto/detail/decltype.hpp
	include/boost/proto/detail/memfun_funop.hpp
	include/boost/proto/detail/poly_function.hpp
	include/boost/proto/detail/preprocessed/extends_funop.hpp
	include/boost/proto/detail/preprocessed/extends_funop_const.hpp
	include/boost/proto/detail/preprocessed/memfun_funop.hpp
	include/boost/proto/transform/detail/pass_through_impl.hpp
	include/boost/proto/transform/detail/preprocessed/default_function_impl.hpp
	include/boost/proto/transform/detail/preprocessed/pass_through_impl.hpp
	include/boost/proto/transform/detail/preprocessed/when.hpp
	include/boost/proto/transform/detail/when.hpp
	include/boost/proto/transform/pass_through.hpp
	include/boost/proto/transform/when.hpp
	preprocess/preprocess_proto.cpp
	preprocess/wave.cfg
patch from develop