Merge pull request #2 from Elzor001/master

Pete
+ ScoreManager [PS]

Implemented ScoreManager which also controls game time. added "touches"
bool to player class. updated collision code to work for centred
sprites.
+ EndGameScene [PS]

added end game scene. Included alive variable to gamescene
+ Rotating turret on tank [PS]

added a second tank sprite, game also gets harder over time
+ added "Play" button to startScreen [PS]
+ HighScore Screen [PS]

added highscore button to startscene, highscorescene with sorting scores
Merge pull request #3 from Elzor001/Pete

+ Pete's branch merge to master [PS]