pip support for wheel

Discussion:

- pip.pep425 refactor to distlib
- remove mention of markerlib dependency
- raise error with setuptools (not distribute)
- priority logic to prefer a wheel for your platform over one with no platform specified
- pip wheel --keep-existing (or --force in reverse); i.e. some way to not redo wheel building. if you don't want.
- cookbook entries
- tests needed
  - use-wheel in requirements files
  - "pip wheel --use-wheel" finding subreqs from a wheel
  - no-deps with wheel installs