Use pip's unpack_http_url and download cache

Using `unpack_http_url` works around issue #87 and makes pip handle the
unpacking logic itself.

The drawback appears to be that the packages in pip-tools cache are not
zipped/compressed anymore.

Fixes https://github.com/nvie/pip-tools/issues/87
Merge branch 'future-pip-unpack-and-download-cache' into local

Conflicts:
	piptools/package_manager.py