Update AttachmentView.java

put image load into background thread.
Update PaintView.java

put bitmap decode into background thread.
Update FeedbackManager.java

put bitmap write/compress into background thread.
add final keywords
Update PaintActivity.java

put bitmap write/compress to background thread.
fix indentation
fix indentation
Update README.md
Merge pull request #50 from yulin2/async-refactoring

put disk access into background thread.