Merge pull request #174 from dreamteam69/master

Garbage collection (remove off-screen entities)
Merge pull request #175 from dreamteam69/master

Stack tracing for debug 24982f867aa0ee56dd5845cd5b0c72ba8275a709
Background class
Merge pull request #176 from ping19/master

Background class
Background setup
Merge pull request #177 from ping19/master

Background setup
Adding background to canvas
Merge pull request #178 from ping19/master

Adding background to canvas
Merge remote-tracking branch 'upstream/master'