v0.2.3b patch notes

This is a very small update to try to increase the difficulty of the game overall by spawning more enemies throughout the run and generally increasing the costs of some planets and ships. 

• Added new backgrounds across the game
• Adjusted rarities and costs of various planets and ships 
• Increased the amount of enemies that spawn over the course of the run on all difficulties
• When a boss is defeated at the end of each system map you gain +1 max ship slots (previously was every 10 rounds)