Return a matched path template during routing

This patch adds the matched path template string into the WSGI
environs in the 'wsgi.matched_path_template' key. The path template is a
string like '/users/{user_id}' that indicates the route that was matched
by the `falcon.api.API._get_responder` method, and this string is useful
in a number of secnarios, including authorization hooks that need to
know the "object path" that was matched so that, for example, they can
look up an appropriate policy/action rule.

For instance, say I have a before hook that does some authorization that
looks at a string like 'users.123.get' and determines if the
authenticated user is allowed to access user 123's profile. Unless that
hook has access to the `falcon.api.API`'s route matching decision, the
hook would have to parse out the request HTTP path and essentially do
the matching against a policy string itself. Why not allow the hooks
access to the already-made routing decision and the match path template
to save the hook from having to re-do this work itself.

Addresses Issue #229