Add support for zlib decompression

[AlstonLin]

bravado-core should be able to handle compressed responses.
These can be detected by the presence of a Content-Encoding header.
Specifically, I'm interested in support for decoding responses compressed by zlib, which have a Content-Encoding: deflate header. This would allow for decreased network latency and bandwidth for users if they want to use compression for their responses.

As the base clients seem to already automatically decompress responses based on the Content-Encoding header for us, we only need to modify the validation code.