Add `_session` to `PackageManager` (required with pip master)

It is a required argument for `PackageFinder` and `get_file_content`.

Tested with pip master (1.6) and 1.5.6.

Fixes https://github.com/nvie/pip-tools/issues/100.
Merge pull request #101 from blueyed/pip-session

Add `_session` to `PackageManager` (required with pip master)