Boost.Build Python port overall status

What works

• Interface between Python and Boost.Jam build engine is finished.
• Project loading and target declaration works
• Top-level code is ported, including getting build request, loading configuration files and projects and building/cleaning.
• A simple project with a 'make' rule loads and builds fine

Short-term goals

• Port remaining core modules.

Medium-term goals

• Design proper error reporting mechanism
• Port all of tools