Update 1.1

Difficulty modifiers added.
Character upgrades capability added.
Achievements added for completing the game without using permanent upgrades (excluding characters).
Character descriptions added.
Bright flash added when the player takes damage.
Damage from the drone squad increased by 20%.
Damage from fire drones increased by 25%.
Damage from the autocannon increased by 15%.
Laser cools down for a longer time.
Critical explosion deals 20% damage from the attack instead of a fixed value.
Bug fixes and minor improvements.