Allow pip paths to use the file:// protocol and allow local files or directories in find_links.

The current pip module is a bit too restrictive with the paths that it will accept (only http/https/ftp), which specifically prevents me from using a local directory of downloading packages for offline installs (http://www.pip-installer.org/en/latest/cookbook.html#fast-local-installs).

I'm not sure if os.path.exists(...) is quite the check you'd prefer to use, but it seems in line with other parts of the module.

Thanks!