Fix for BOOST_JOIN errors.

Hello :-)

Fist things.. nice project!! i like avogadro and i use it for teaching and not only :+1: 

Second things... i'm trying to make a formula for homebrew in osx.. however i got some compilation error such as BOOST_JOIN using boost 1.56 and i have seen that there are a lot of bug reports without a real fix. So, i've resolved the problem fixing pythonengine_p.h, pythonextension_p.h, pythoninterpreter.h, pythonscript.h and pythontool_p.h. Here i submit to you this pull request in order to integrate this if you are interested. 

Best Regards

Giuseppe Marco Randazzo