Added BasicEntity file, updated App to allow placing of circles

include/App.h
  - Added onMousePress()
  - Made the following functions virtual
     = onInit()
     = onClose()
     = onExit()
     = pollEvents()
     = onMousePress()
     = render()
  - Added array of POINTERS to BasicEntity to render circles
     = POINTERS due to polymorphism arrays not possible in C++
  - Added size and capcity of said array

include/BasicEntity.h
  - Created basic superclass of entities
  - Draws nothing

include/CircleEntity.h
  - Created entity for a circle
  - Allow for drawing of circle by containing sf::CircleShape

src/App.cpp
  - Edited onInit() to initialize the entity array
  - Edited onExit() to delete the entity array
  - Defined onMousePress() to add circles on mouse press
  - Edited render() to render the entities

src/CircleEntity.cpp
  - Defined constructor, setPosition(), setFillColor()