Boost.Build Python port overall status

What works

• Interface between Python and Boost.Jam build engine is finished.
• Project loading and target declaration basically works
• A simple project with a 'make' rule loads and builds fine

Short-term goals

• The top-level Jam code is not ported yet. In particular this means that user-config.jam is not loaded yet, and command line build request is ignored.
• Some details of project loading are to be ported yet -- in particular the 'project' and 'using' rules.
• The 'make' function has a dummy 'self' argument to workaround some wrong details of making rules available to Jamfiles.