Implemented Rendering components hierarchy.

From now on, every components that renders something into screen needs to inherit from components::Renderer. This will let us have a collection of components::Renderer to which we can make render using render(Vector2D& position) instead of having different collections for each type of rendering component.

This is a step toward implementing parallax scrolling.
Added Parallax Scrolling functionality.

The Parallax Scrolling is archieved by using a float from 0.0 (static background layer) to 1.0 (normal layer). This factor is multiplied by the camera correction vector when the position of the objects is calculated.
Merge branch 'rendering' of https://github.com/Poles/Poles into rendering

Conflicts:
	.gitignore
	CMakeLists.txt.user