1.0.0alpha1 - Cache coherence + breaking changes

EntityX has switched to a more cache-friendly memory layout for components. This is achieved by requiring the use of assign<Component>(arg0, arg1, ...) and removing assign(component). This allows EntityX to explicitly control the layout of components. The current lyout algorithm reserves space for components in chunks (8192 by default).

This change also necessitated a move away from the use of shared_ptr<> for components, which I had never been that pleased with anyway. Replacing it is a very lightweight ComponentHandle<C> smart pointer. This checks for validity of the entity associated with the component, and validity of the component itself. It also allows future iterations of EntityX to do even more interesting things with memory layout if desirable.