Avoid reinstalling packages by elpa-get

This PR introduces the following changes:

1. By default, elpa-get does not reinstall installed packages.
2. --reinstall option is added to force reinstall.
3. site-start.el saves the location of it to virtualenv-site-lisp so that elpa-get does not need to know $VIRTUAL_ENV.

The last part solves the same problem solved by #1.  I think this implementation is better.