Add vendor library management support to pip

Do a lookup for config files, looking from the current directory for pip.conf/.pip.conf/pip.ini
Do some limited %() substitution in config files
Handle empty lines in config files better

This is a refreshing of https://github.com/pypa/pip/pull/491 (with bad commits removed)

Original description:



This pull request makes pip look in the current and parent directories for `pip.conf` and loads it. Also `%(here)s` gets substituted in config values.

The motivation here is to make directory-local vendor libraries easier, with a config file like:

    [install]
    install_option = --install-purelib=%(here)s/vendor
        --install-platlib=%(here)s/vendor-binary
        --install-scripts=%(here)s/vendor/bin

Or something like that... the idea isn't complete. Feedback appreciated.