Expose Handler, so you can do things at the http.Handler level

I wanted to add a global http level handler filter, like a [gzip filter](https://github.com/darkhelmet/webutil/blob/master/gzip.go)

This is a simple change that lets you override the way revel builds the handler, so you can make a chain of them to handle things on the http side of things.

Not sure how I could/should go about testing this, but it worked for me locally, even with reload.

My simple test case in my `app.go`

```go
func init() {
    revel.BuildHandler = func(f http.HandlerFunc) http.Handler {
        return http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
            w.Header().Add("X-Revel", "Hello")
            f(w, req)
        })
    }
}
```