Add set_package_preload.cpp and function_introspection.cpp to Jamroot.

Hi, first off thanks for your fork of luabind, it's nice that there's a version that works with clang and lua-5.2. Second, I encountered a problem on OSX 10.8/clang building using 'bjam toolset=clang install': there were missing references to some subroutines. I did a find/grep for them, and this revealed the two files mentioned in the title. Once I added these to the rest of the cpp files in Jamroot, the project built without any errors. 

I'm new to open source contribution, so I don't know how this pull business works,  but basically it's just two additions to Jamroot. Let me know if you want to get my changes :)