[boost] [fiber] new version

I've uploaded a new version of boost.fiber (0.4.2) to boost-vault. The context/stack swapping is implemented in assembler for i386 and x86_64 (see documentation). Because of the limitations of boost.build you have to compile bjam with python support (see docu of boost.fiber). Please note that I plan to refactor boost.fiber into boost.continuation and boost.microthread. Oliver