remove unused Entity class
systems now have a reference to entities instead of anonymous components
Merge branch 'master' of https://github.com/maarten990/Objectless
implement positions

make the GraphicsManager draw textures at the position specified by
their corresponding entity's Position component
add keyboard movement code sample
add call to SDL_RenderClear to clean up every frame
fix memory leak when destroying entity
simplify System::notify_destroyed
update documentation
fix erroneous use of vector::erase
split game and engine code

the engine is now linked as a static library