Improving ECS and adding documentation.

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+# Entity Component System (ECS)
+
+## Structure
+
+### Entity
+
+The `EntityManager` is the class that manages the entities, by allocating and
+deleting them as necessary. The `Entity` itself is just a handle, and does not
+contain any logic or data. It is just used to refer to the `Entity` when doing
+operations on it.
+
+### Components
+
+These are just data that is associated with Entities, and it gets processed
+by the Systems when these are updated. The systems only process a few components
+at once, which separates the logic and makes it easy to develop the Systems and
+Components orthogonally. 
+
+### Systems
+
+These process the Components and update their state. This completely separates
+the data from the logic, and means that they can be tested independently. New
+systems can be created by inheriting from the `BaseSystem`, which provides an
+interface which then enables that system to be added to the main `Engine`
+through different types of `Spaces`.
+
+## Implementation
+
+The Entity is just an `unsigned int`, which has a `typedef` called `Entity`,
+which is also what all the functions return. 
+
+The link between the components are managed by a templated `ComponentMapper`
+class, that links every entity with it's component. This way it is also to find
+out if an entity is part of a specific component, and that entity will never be
+processed in the system that only targets that component, which makes it much
+more efficient.