Finish return data format using json and define return codes (may still need improve). Not finish test. Need refactor book model.
Merge remote-tracking branch 'origin/WLB' into WLB

Conflicts:
	Server/application/controllers/download_controller.php
	Server/application/controllers/search_controller.php
	Server/application/controllers/user_controller.php
	Server/application/models/book_model.php