It differs from the legacy router:
* it provides somewhat granular errors: "path not found", "method not allowed".
* it handles matching routes with extensions (e.g. /books/{id}.json)
* it handles path patterns with a different syntax (e.g. /params/{x}/{y}/{z:.*})
NewRouter creates a gorilla/mux router.
Assumes spec is .Validate()d
Note that a variable for the port number MUST have a default value and only this value will match as the port (see issue #367).