Generate cache key based on req.originalUrl instead of req.url.

Fixes issue #18

req.url may be a relative url from a given mount point, so there
could be key collisions in an app with same-named relative paths
mounted on different roots.

req.originalUrl contains the full url including the mount point.
Fix typo in README
Merge pull request #19 from barwin/req_original_url_no_collisions

Generate cache key based on req.originalUrl instead of req.url.