Refactoring - Providing constant interface for the asynchronous data.

[Issue]
  Images, audio, and Shit-JIS texts are processed in ad-hoc manner.

[Solution]
  By introducing a new class, MediaLoader, these data are now decoded, loaded, and retrieved in constant manner.

[Affected parts]
  QuickSWF and SWFCrew.
Added comments.
Eliminated a small bug in the mime-type string manipulation.
Moved the PersistentCueListener#cue() method calls to within _update().
Abandoned the option 'alias' for MediaLoader#load() due to simplicity.
Changed not to make requests if the data is already requested.
Fixed a bug injected on the previous refactoring.
fixed definedtext (still has an issue about descendant).
Modified with correct initialization values for better performance.
Merge pull request #31 from kuu/master

Refactoring - Providing constant interface for the asynchronous data.