made uri immutable.  removed uri forwarding methods

Removes the methods that were just a wrapper around URI and added a toURI method.

This keeps `TachyoneURI` more inline with Hadoop's `Path` and gives us less we need to maintain.

Also made the internals fully immutable.  It was before after construction, but made it more explicit.
Why don't we just keep it as 'uri.getHost()' ?