[python] don't include site-packages in python path

This is very error prone as add-on may 'silently work' if required dependence are installed system-wide. It's basically impossible to test if your add-on really works without a clean python install.

I have no idea why site-packages was [added](https://github.com/xbmc/xbmc/commit/c4fd22dd326af8a82c50d6f7d0df85a6518aa14b) in the first place. May completely break something.


I think this would fail on windows/osx/android as there is no PIL in site-packages. _*.so *.py from Imaging shipped in addons/script.module.pil/lib ?

better:
```
try:
    import imp
    from site import getsitepackages

    fp, path, descr = imp.find_module('PIL', getsitepackages())
    try:
        imp.load_module('PIL', fp, path, descr)
    finally:
        if fp:
            fp.close()
except:
    pass
```

Ignore me if that's not the case, I can't test on win/osx/android. just posting this as a warning that PIL.py should never fail

other than that, it works on linux (openelec tested with cdart manager)