Previous commit added means to decide whether client asks for compression and with which algorithm.
This patch adds actual compression of responses based on zlib library.
For now only string (not chunked) responses are compressed.
Several previously defined tests start to pass.
This is an initial patch to add support of Alternator's compressed responses.
The actual compression (gzip,deflate) will be added in the following commits.
The main functionality added in this commmit is parsing of Accept-Encoding header,
that indicates compression algorithms supported by the client.
In this commit we add also configuration parameters of response gzip/deflate compression.
They allow to enable/disable compression, set level and a size threshold below which a response is not compressed.
With current implementation it is possible to decide a compression for each response, but it is not used yet.
