Add better coverage reporting to pex

Adds the ability to test the code running within the .bootstrap directory of
a pex environment, which otherwise does not get run by pytest-cov.  Also
adds the ability to integration test pex/bin/pex.py.

Testing Done:
tox
tox -e py2-integration,py3-integration,combine
open htmlcov/index.html

Reviewed at https://rbcommons.com/s/twitter/r/836/