Middleware for compression

[aturon]

A good test of the middleware system would be to implement gzip compression as middleware (transforming the body and headers on the way in/out).

[yoshuawuyts]

This would tie in nicely with rustasync/team#52

[fairingrey]

I took up this issue up roughly two weeks ago through #194 and progress is almost finished. There are some things I want to wrap up before I submit it for review.