Make pip-accel with pip 1.4 (just released)

This pull request groups the changes required to get pip-accel working with pip 1.4 (released two days ago). Unfortunately two out of the three changes kind of look like bugs in pip from my perspective. But maybe that's just me; I guess pip shouldn't "officially" be used as a Python API; the only advertised use case is the command line program (AFAIK).