Moved gzip middleware into its own package

Trying a different format where the middleware is a type that satisfies http.Handler

middleware/gzip.go → middleware/gzip/gzip.go

-package middleware
+package gzip
 
 import (
 	"compress/gzip"
 	"io"
 	"net/http"
 	"strings"
+
+	"github.com/mholt/caddy/middleware"
 )
 
-func Gzip(p parser) Middleware {
+// New creates a new gzip middleware instance.
+func New(c middleware.Controller) (middleware.Middleware, error) {
 	return func(next http.HandlerFunc) http.HandlerFunc {
-		return func(w http.ResponseWriter, r *http.Request) {
+		gz := Gzip{next: next}
+		return gz.ServeHTTP
+	}, nil
+}
+
+// Gzip is a http.Handler middleware type which gzips HTTP responses.
+type Gzip struct {
+	next http.HandlerFunc
+	// TODO: Compression level, other settings
+}
+
+// ServeHTTP serves a gzipped response if the client supports it.
+func (g *Gzip) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 	if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
-				next(w, r)
+		g.next(w, r)
 		return
 	}
 	w.Header().Set("Content-Encoding", "gzip")
 	gzipWriter := gzip.NewWriter(w)
 	defer gzipWriter.Close()
 	gz := gzipResponseWriter{Writer: gzipWriter, ResponseWriter: w}
-			next(gz, r)
-		}
-	}
+	g.next(gz, r)
 }
 
 // gzipResponeWriter wraps the underlying Write method