BuildHandler, 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.