Make `pip show` show much more metadata

New fields:
  - `Metadata-Version`
  - `Summary`
  - `Home-Page`
  - `Author`
  - `Author-email`
  - `License`
  - `Entry-points`

Sample output:

    $ pip show pytest
    ---
    Metadata-Version: 1.1
    Name: pytest
    Version: 2.6.4
    Summary: pytest: simple powerful testing with Python
    Home-page: http://pytest.org
    Author: Holger Krekel, Benjamin Peterson, Ronny Pfannschmidt, Floris Bruynooghe and others
    Author-email: holger at merlinux.eu
    License: MIT license
    Location: /Users/marca/python/virtualenvs/tmp-4e2169280f5cbd16/lib/python3.4/site-packages
    Requires: py
    Entry points:
      [console_scripts]
      py.test = pytest:main
      py.test-3.4 = pytest:main
Yeah, I thought of this as well, but I didn't change it because the existing code uses `%` and I wanted to be consistent and keep the diff small and easy to read.

Also, I know there are some other PRs that are doing stuff with the `show` command and I don't want to create merge conflicts.