1. Added a SaveLoad class which saves/loads to/from the PSPs file system
a binary file which contains the player's game progress
2. Implemented the XP system; now players can see their available XP on
the Title Screen
3. Implemented to Score system; the Game Screen displays the player's
current score in the top right corner (the player's score increases
every second he is alive and relative to the enemies' speed)
Fixed a bug where the score would not reset between sessions if the
player pressed Start to exit the current session.
Added an Energy system to the SkillsSystem class
Added a Skill Screen where the player can manage his skills. The player
can level up skills by spending XP points he gained by playing. A
maximum of 10 skill points can be invested in any skill until the Drive
Core becomes fully loaded. This forces the player to strategically
choose his upgrades in order to help him survive as long as possible.
At any time, a skill point can be refunded for a fraction of its
original cost (except for the first skill point of every skill, which
has an XP cost equal to its refund value).