Displays info about [un]installed versions

This is a variation on https://github.com/pypa/pip/pull/2207 that doesn't write the output in `package_name==version` format, because @dstufft had some reservations about using that format and then people getting accustomed to it.

Displays version when installing and uninstalling -- e.g.:

    Successfully installed pyrepl-0.8.4 pytest-2.6.4

and:

    Uninstalling pyrepl-0.8.4:
      Successfully uninstalled pyrepl-0.8.4
    Uninstalling pytest-2.6.4:
      Successfully uninstalled pytest-2.6.4