Add Screen Navigation

This request contains the code for navigating from a main menu and for respawning.

There's no scene included in this (or there shouldn't be) but making the main menu work is easy - just make a new scene and attach the Navigator script to an empty game object (it's found in the Main Menu folder inside the Scripts folder hierarchy).

The UI Controller should already have the HealthMonitor script attached to it, if not then...yeah. The way it works is that it monitors the players current health; once it reaches zero it displays a menu prompting the user to respawn or quit (Quit won't work in the web player).

Unfortunately, health for some reason isn't working on my machine with the current build, so someone else needs to test the health monitoring.