Install pass.py under site-packages instead of /usr/bin

Since it is not a binary but a python lib, it's better to install it under
site-packages. That way, people that install the package through setup.py need
to specify only the following in the python-keyring/keyringrc.cfg file:

[backend]
default-keyring = pass.Keyring
Merge pull request #3 from tampakrap/setuppy2

Install pass.py under site-packages instead of /usr/bin