Added SceneCredit and SpriteCredit.

[NEW] Added SceneCredit.
[NEW] Added SpriteCredit.
Fixed the wrong y position of texts in SpriteCredit.

[NEW] Added debug information to SpriteCredit.
[FIX] Fixed the wrong y position of texts in SpriteCredit.
The SpriteCredit now can play pictures and can be controled by SceneCredit.

[NEW] The SpriteCredit now can play pictures and can be controled by
SceneCredit.
[ENH] Now the SpriteCredit can stop at the end of the contents and wait
for a while.
Improved the code quality of SpriteCredit.

[NEW] Added SpriteCredit.pause().
[NEW] Added SpriteCredit.resume().
[NEW] Added SpriteCredit.setFadespeedFrames().
[NEW] Added SpriteCredit.setTransitionFrames().
[MOD] Changed the initial position of ScrpteCredit.
[ENH] Improved the code quality of SpriteCredit. Now it uses coroutine
to update.
Merge branch 'scene-credit'