New enemies and more gameplay improvements

Here are the new features of version 0.4.0 (most of this features were suggested by https://twitter.com/tricky_fat_cat):

• New enemies
  • Triangle that points to the player for a short period of time and then it increases it's speed drastically and goes in a straight line.
  • Square that does a small dodge while it's spawning an enemy

• Updated enemies
  • The yellow ball now bounces pointing to the player always.

• Power Ups
  • Power ups have a slighly reduced cooldown.

• Sound
  • Background music has been added.