Adding a Rhino mini-boss

Created a rhino mini-boss that the orcs have to avoid. Also, you have to build a path for them. The rhino pauses briefly at the end of the path for about 4 seconds. So getting all orcs across safely is all about timing. 

In the future, I would like to build upon this AI mechanic, as well as, add a timer to the game. 

The mini-boss occurs after a player completes the tutorial.