Fixed 3rd-party package installation for namespace packages and overriding built-in libraries

Replacing sys.path.insert() with site.addsitedir() makes it possible to import namespace package dependencies.
I changed the API to match the Darth package, so ```index``` is now an optional argument defaulting to 1. I don't think that the ```path``` and ```dir``` options need to be distinguished with separate functions, as ```addsitedir()``` will take either anyway. Accordingly I simplified the ```appengine_config.py```.

I also added an ```insertvenv()``` function to match Darth's functionality.