Make the Core::Spaceship#warp documentation more specific
Make Core::Spaceship#initialize take valid direction representations
Add examples with #velocity to the Core::ParallaxBackground spec
Improve the documentation of Core::Spaceship
Add more attribute getters to the Core::Spaceship class

Those would be handy later on and it would not be cool to be reaching
for the constants in Core to get that information.
Add more Core::Spaceship explanations in the documentation
Add spec for Core::Spaceship