Now jumping really works. No infinite jumping in the air anymore.
Added some info around jumping methods.
Changed MOVEPX indirectly to 15 px.

The MOVEPX can be changed in two ways:
1. Changing the MOVEPX value itself.
2. Changing the FPS, the MOVEPX doesn't change, but the visualising of
the movement itself goes faster. For example: we have MOVEPX = 10 and
FPS = 30. If we change FPS to 45, the MOVEPX 'changes' indirectly to 15.
Added some info about how to change MOVEPX.
Merge branch 'master' of https://github.com/ikhebgeenaccount/SideScroller.git