alter :path-info as well as :uri

When changing the path of a request, be sure to change :path-info as
well as :uri.

Ring actually looks at :path-info *first*. See
https://github.com/ring-clojure/ring/blob/1.2/ring-core/src/ring/middleware/file.clj#L21
and
https://github.com/ring-clojure/ring/blob/1.2/ring-core/src/ring/util/request.clj#L32-L36

So, when we're running on a server that supplies :path-info, we have to
be sure it is correct, or else we'll end up either 404'ing (bad) or
serving the unaltered content (possibly worse).
load v8 at the last possible moment

Since v8 is not always available, don't load it when the file is loaded,
wait until someone calls into us with {:engine :v8} set.
Merge pull request #59 from kbaribeau/master

alter :path-info as well as :uri