Add prebuilt python library.

This adds the ability to include eggs & wheels (well not yet; needs a pex upgrade) in pexs generated by buck. This can be extremely useful when trying to include third-party libraries that don't easily lend themselves to buck-ification (e.g. gevent).

The syntax looks like

prebuilt_python_library(
    name='gevent'
    srcs=['gevent-1.0.1-py2.7.egg'],
)

python_binary(
    name='my_module'
    main='main.py',
    deps=[
        ':gevent'
    ],
)