make README as markdown
arrange directory structure
add new Updater skeleton
add anchor.  implment new downloader
add comments for downloader
sort import statements
use existing extractor
fix extractor: support some failure case
suppress exceptions in extract()
finish to simple cases
implement messages.
implement progress callback for extractor
implement progress callback for downloader
finish to impl progress bar
prepare for message localization
fix readme
fix batch files
fix progress bar
Merge branch 'develop'