Coins now spawn at an increasing rate

Coins spawn and expire somewhat randomly, using a random range to set the values for each instance. 

Previously the min and max ranges were hardcoded (and set to be quite slow for ease of testing) but now they increase over time. The game is split into four timed "difficulty" segments; every time you reach a new segment (currently every 15 secs) the spawn and expire rate increases. By the last segment, it's pretty tough to hit all the coins. Finally a bit of a challenge!