WIP/TBD: 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!
I am not sure if this is such a bad thing (with regards to disk space usage).

It looks for `['pydist.json', 'metadata.json']` in wheel archives - I found the latter to be used with raven-python.

It adds a warning if no dependency information could be found, and removes the old handling for VCS urls, which appears to have defaulted to setup.py anyway?!:

-                    deps = self.read_package_requires_file(path)
+                    logger.warn("Package has no dependency information: %s" % (path,))

Fixes nvie#87