PLUS : Made level 10 final level

* made some preparation for an endless level
BUG FIX : Saving progress

* Previously when you were finishing a level, the max level would be the
level you just passed. Let's say you unlocked all the levels until level
8, than you go replay level 2. If you win level 2, the biggest level
will be level 2, so you will have the option of choosing level 1-3 to
play now.
PLUS : Working Endless mode

* Endless mode now available, giving you the ability to purchase any
towers that you have unlocked throughout the main 10 levels