BUGFIX: Corrected an issue where you could add multiple screens with the same name
All screens must now set _screenName in their constructor

Signed-off-by: Lewis Storey <iamlewisstorey@yahoo.com>
Got the initial stuff completed for the power ups
Need to make textures and stuff and test they work
Should probably create the rocket launcher at some point

Signed-off-by: Lewis Storey <iamlewisstorey@yahoo.com>