Call initInflate and initDeflate exactly once (extension to pull-request #6)

@dimbleby could you take a look at these changes and see if they work for you? I took your changes and improved them a bit to integrate better with the other `pipes` libraries. In particular, these changes are compatible with the newest `pipes-parse` (though we don't need a dependency on that library).

If these changes are fine I'll push them to Hackage as `pipes-zlib-0.4.0`.

Also, is it OK with you if I add your name to the `PEOPLE` file? I don' know your name, though, maybe you can help me there :)