Python 3 support

Plugin can now be compiled to work with either python2 (tested on python2.7) or python3 (tested on python3.2).
translator.c and python_interface.c have both been duplicated in 2 src subdirectories (py2 and py3) due to different naming of python embedding functions depending on the version of python.
Modified autogen script to check python version and modify Makefile accordingly to compile with the user's python version.
Made further changes to python/apertiumInterfaceAPY.py so that it works with both python2 and python3.
Changed README and doxygen mainpage to reflect new changes.