Make some practical tweaks for projects which bootstrap their trust chains by checking a tarball of peep into their source trees.

* Start supporting versions of pip back to 0.6.2 (released in January 2010).
  This way, you can deploy trustworthily on old versions of RHEL just by
  checking a tarball of peep into your source tree and pip-installing it; you
  don't have to check in pip itself or go to the bother of unpacking the peep
  tarball and running ``python setup.py install`` from your deploy script.
* Remove the explicit dependency on pip. This is so a blithe call to
  ``pip install peep.tar.gz`` without ``--no-deps`` doesn't go out and pull
  an untrusted package from PyPI. Instead, we scream at runtime if pip is
  absent or too old. Fail safe.