Use more generic way of creating ParallaxBackground object
Update the Game class I use to test the changes
Remove unnecessary blank line
Restrain from using methods as hashes

Setup Core constants for now.
Try to kick gosu out of the core, first step - time management

The Core should be in no way dependent on gosu. After all it uses only
three methods from that gem...
Fix bang methods and #magnitude_squared
Fix #/ method, now does fdiv instead of integer division
Use is_a? or kind_of? consistently
Eradicate Gosu from the Core
Further separate logic and gui

All the knowledge of when and how to reset the offset should be concern
of the renderer, not the core object itself.
Move the constants from Utils to Core

In Utils are kept more general stuff, like Vec2D. Those constants are
more specific for what I'm doing and are not universal.
Use the constants in Core and remove unnecessary instance variable
Use more idiomatic ruby instance variable name
Remove unnecessary instance variable and reader
Remove require of deleted file
Reorder some of the methods
Give better name to method
Add default speeds so bullets could be spawn by Spaceship#shoot

Otherwise I should have had info about the bullet speed in the spaceship
instance.