Only install stable releases by default

* Adds the --pre flag to pip install to specify that you wish
  to install pre-releases
* If the version spec contains any pre-releases then continue to
  install pre-releases
Test the Stable/Pre-release distinction in PackageFinder
Document the logic behind the Pre Release/Stable distinction
Switch to using a fully vendored distlib inside of pip.vendor

* Includes helper scripts to maintaing pip.vendor
Merge pull request #834 from pypa/feature/stable-by-default

Only install stable releases by default