Normal mode added and other improvements

Some players have found Hard mode to be too difficult starting out...and no one wants to admit to needing Easy mode...so I have added a Normal mode to each level--easier than Hard, harder than Easy, as one would expect.

I've also modified some of the settings on the early levels (mostly made them shorter), to get players to the more complex levels sooner.

The biggest changes in this update, however, are behind the scenes. The game has been updated to Unity 2018, I've implemented and object pooling system for the enemy spawners, and have greatly streamlined the code. These changes may result in small improvements in performance, but their real value is in making Willowisp VR much easier to maintain and update.

Willowisp VR will be leaving Early Access soon as I move on to other projects, so if you have any suggestions for improvements, notice any bugs, or have any other comments, any feedback is much appreciated!