zlib interface semi-broken

[Travis]

Hello all,

The zlib interface does not indicate when you've hit the end of a compressed stream.

The underlying zlib functionality provides for this.

With python's zlib, you have to read past the compressed data and into
the uncompressed, which gets stored in Decompress.unused_data.

As a result, if you've got a network protocol which mixes compressed
and non-compressed output, you may find a compressed block ending with
no uncompressed data following until you send another command -- which
a synchronous (non-pipelined) client will not send, because it is waiting
for the [compressed] data from the previous command to be finished.

As a result, you get a protocol deadlock.

A simple way to fix this would be to add a finished attribute to the
Decompress object.

However, perhaps this would be a good time to discuss how this library
works; it is somewhat awkward and perhaps there are other changes which
would make it cleaner.

What does the python community think?
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