Repurpose pip.__init__.bootstrap()

Previously `pip.__init__.bootstrap` was dead code for the removed `install-pip.py` file. I saw an opportunity to move `get-pip.py`'s bootstrap function into this, which simplifies `build-installer` / `get-pip.py` a little.

functions' output are now correctly returned up to `sys.exit()` so any errors will return an error code to the os properly - which should fix #1535.

This could also allow in the future (if one desired), the creation of a simple Command alla `pip bootstrap`, or e.g. `python pip-1.6.0-py2.py3-non-any.whl bootstrap` to bootstrap pip from its own zip file / wheel (*avast, a wheel being used as a zip file!*).