remove support for requirements files

Supporting requirements files was unnecessary and oddly allows passing
additional arguments to the pip subcommands. Stick to packages only for
simplicity.

In practice, most packages declare their dependencies, and pip will
recursively install these dependencies.
py2k compatibility
discover and pass index url when building wheels

Discover the HTTP URL to the S3 bucket index pass it to `pip wheel`.
This prevents pip from building wheels that already exist.
style
release: 0.2.0