Add achievement notification

Addresses #20.
Adds a ! notification when the player gets an achievement.
Display clicker mps correctly

Addresses #30.
Clicker mps is now displayed with mps_factor taken into account.
Add 9 more achievements

Addresses #25.
Adds 9 more achievements to the game.
Fix saving of achievements

Addresses #25.
Achievements are now saved and loaded properly.