Bug fixes and new enemies!

-Fixed 'zombie hits once bug' by adding Player invincibility frames and
animation
-Player life was changed to 3 (Thanks unity 4.6)
-Main menu loads upon death.
-Fixed bug where zombies could attack and be moved by player in their
death animation
-Items now disappear after 6 seconds.
-Large Zombie and Fast Zombie have been added.
-Each zombie now has a unique sprite
-Overall speed of zombies was increased, and zombie healths were
re-balanced.