Add utility for normalizing vectors.

* Expose earth radius and reduce it.
* Add methods that normalizes the scale of vectors to a specific value.
Merge pull request #3 from adam-borzecki/feature/asteroid-generator

Feature/asteroid generator
Merge branch 'develop' of github.com:adam-borzecki/asteroid-defense into feature/miner-missile-interaction

Conflicts:
	asteroid-defense.xcodeproj/project.pbxproj
	asteroid-defense/Application/Projectiles/Nuke.m
	asteroid-defense/Space.m
	asteroid-defense/ViewController.m
Add contacts between nodes.

* Added collisions between earth and asteroid.
* Added collisions between nukes and asteroid.
* Added miner classes.
Initial asteroid spawner, update gravity code
Remove renamed file
Refactored collision logic in Space to a manager.
Fix conflicts
Merge Conflicts