Automatically download and cache new pip/setuptools versions

[ulope]

With the faster release cycle pip and (to some extent) setuptools nowadays enjoy it's become increasingly likely that newly created virtualenvs will contain an outdated pip version.

I would suggest that virtualenv should stop bundling a specific pip/setuptools version and instead download and cache the respective current version.

Later if later no internet connectivity is available, the cached version can be used, otherwise a version check can be performed and the user be prompted to update.

[ulope]

Awesome, thanks :)