Update singleton.h

Changing to Meyer's Singleton Implementation as it's also lazy initialized but simpler and thread-safe as stated on C++ standard since C++11 ( §6.7 [stmt.dcl] p4 ).
Update singleton.h

Fixing error created while changing to Meyer's Singleton Implementation.
Changing Variable Names

Changing variable names to follow the naming convention used.
Merge branch 'master' of https://github.com/TiagoRabello/game-programming-patterns into singleton

Conflicts:
	code/cpp/singleton.h
Tweak Singleton to explain static variable.