Gzip support / A way to add a step before Response is sent

[dbrain]

As someone who likes to save their megablibs, I'd like soggy to be able to compress responses before/during sending. This one might be a bit fiddly.. at the moment all that's really happening are calls to ResponseWriter.Write(..), may need to add a layer on top of that.

Try to make this generic so can be done to a response before its sent through.