get-pip.py updates (due to vendoring pkg_resources)

this has `pip.bootstrap()` installing setuptools, and simplifies the install instructions.
I tested using the fact that `get-pip.py` accepts arguments. 
I ran it using `--find-links` against local pip/setuptools wheels.

I debated whether to have the setuptools install contingent upon `pkg_resources.get_distribution` checks, but decided an import check was sufficient and easier.

this assumes we will:
1. run `contrib/build-installer` to generate `get-pip.py` with the finalized pip before release
2. release a pip wheel to pypi