Make Bundler to respect pathname part of ROOT_URL

If the ROOT_URL environment variable is set the
Bundle.prototype._clientUrlsFor() method should append the pathname
to urls returned

	modified:   tools/bundler.js
DDP default url should respect the ROOT_URL

If the ROOT_URL environment variable is set the
default DDL url must be equal to pathname part
of the ROOT_URL.

	modified:   packages/livedata/client_convenience.js