Save the Hydras

This update I've finally added the second water tower, the Hydra! The Hydra fires multiple water beams that each target a different Hero, and should allow for some fun setups :)

I did a few balance tweaks here and there, as well as some small ui improvements. Biggest would be increased lives on Easy and Normal, making those quite a bit more forgiving.

Most importantly you now have access to the save game feature, between waves you can save the game, to resume later via the main menu. (Or when you die, which never happens to you ofcourse)

PS The credits were also updated to mention everyone who has been playtesting and giving advice. Thanks everyone :D