Support for downloading wheels from PyPI

[takluyver]

The include section has a new pypi_wheels= option, to be used like this:

pypi_wheels= PyQt5==5.6
    sip==4.18

Open questions:

• I included support for getting the latest version of a package by leaving off the ==version, but I didn't document it yet. I'm thinking of taking that feature out, and requiring the ==version specification all the time. This makes what you're building with explicit and consistent, and simplifies the code.
• It only looks at PyPI. Possible extensions include paths to local .whl files, and setting the URL for an alternative PyPI-like server to query.
• Currently the .dist-info and .data directories in the wheels are ignored. These should not normally be needed in an application context, but sometimes they might contain bits we would want to preserve.

[takluyver]

I did change the code to require explicitly specified ==version on wheels to download.