0.5.0

• breaking changes
• rewrite widget code
• tweak style
• tweak DOM structure
• tweak APIs and Events
• tweak uploadr object structure:
• Use uploadr.viewer and uploadr.uploader for widgets, uploadr.ext for providers.
• rewrite all demostration page
• update document
• add block-based widgegt
• support file extension limitation
• add loader in viewer
• support thumb placeholder for non-image files
• i18n support
• native provider: check and fallback file ext used