fixed compile error

EntityManager wasn't declared in Entity.hpp
error didn't occur with MSVC
removed a unwanted recursion in the constructor of Game

call to Game::getHandle() in Game::Game() caused an infinit recursion loop
resolved by moving some code from Game::Game() to the new function Game::init()
this didn't occure with MSVC
activated additional warning messages for debug builds
reordered initialisations to fix some [-Wreorder] warnings
removed unused variable 'origin'
added braces to avoid ambiguous ‘else’
added class DebrisChunk
added static function 'spawnDebris' to DebrisChunk
now debris is spawned when an asteroid get destroyed

also added a graphic for a chunk of debris
changed default value for amount in DebrisChunk::SpawnDebris to 30
added code to handle state of running and only update DebrisChunk if running is true
shortened texture acquisition in Spaceship like it is in the other classes
debris is now also on destruction of the spaceship spawned
modified CMakeLists.txt so the res folder and the config file are now copied automatically to the build directory
ported project to newest versions of SFML and Thor
edited README
added a button manager system which eases the creation of menus and enables control via keyboard in menus
documented ManagedButton and made it posible to instantiate it without a position and set the position afterwards
modified button positioning behavior of ButtonManager
documented ButtonManager