Make constructors public
Merge pull request #20 from pandamicro/v3

Make constructors public
Merge pull request #21 from cocos2d/v3

Sync Cocos2d-x v3
Merge pull request #22 from cocos2d/v3

Sync Cocos2d-x v3
Merge pull request #23 from cocos2d/v3

Sync Cocos2d-x v3
Feature #4977: Create a AAssetsManager to start the implementation
Feature #4977: Add functions and comments
Feature #4977: API fix
Feature #4977: Add manifest parse logic
Feature #4977: Update APIs and property types
Feature #4977: Add Downloader class
Feature #4977: Add DownloaderDelegateProtocol delegate functions in AssetsManager
Feature #4977: Add Thread support in Downloader and make the download process work
Feature #4977: Modify const char* parameters to const std::string &
Feature #4977: Improve download process by moving assertion out of download thread
Feature #4977: Modify const char* parameters to const std::string &
Feature #4977: Permit to create directories recursively in writable path
Feature #4977: Refactor delegate APIs to provide more information
Feature #4977: Use event dispatcher to notify loaded event
Feature #4977: Add Manifest class to host logic related to json parser and manifest management