Update README with correct placeholders in URL.
Use `knownPaths` to return the list of paths in GetRouteList.
Add a few more methods for future introspection to the Router.

The HasPath method check if a path is know by the router and return a
boolean.

The RemovePath method is useful to remove dynamically a path from the
router.

The GetAllRoutesByMethods returns a list of routes that match a given
method.

I'm not sure I'll keep all those methods available since there's no
immutability on the routes, and I don't want the user to mess with
them.  We will see later.
Add a method to get the HTTP methods supported for a path.
`BuildRouter` return a reference to itself.
By calling AddOptions on the router, a route for OPTIONS is added

The AddOptions method on the Router creates a route to respond to the
OPTIONS request, and list all the supported HTTP methods for the given route.
Generate route to respond with 405 when the HTTP method is not existing.

When the user call `AddNotAllowed` on the router, for each path, a new
route will be added for each HTTP method not existing.  The new route
returns a 405 HTTP error and a header containing the list of allowed
HTTP methods.
gofmt