Add bullet sounds and background music assets
A spaceship should provide information about the ammo it is using
Change the ammo of the `:eagle` Spaceship to `:crusher`
Attach the background music to the map resources
Add the new resources in the Resources module constants
Add functionality to Loader to be able to load longer songs
Fix stack level too deep caused by the documentation
Fix the names of the sound and music assets
Add SoundManager class to deal with all the sound effects
Improve Core::Spaceship#shoot's documentation
Split the `:shoot` spaceship action into two to be more informative

So we know if the spaceship has just pulled the trigger or has pulled
the trigger and fired a bullet.
Change the Spaceship spec to reflect the changes