add mail handler
add database to project

now database is not connected to the model yet.
use taskqueue to process every client's subscribe
add TODO list

1.add todo list
2.remove login from app.yaml so that more client can use this service
 Changes to be committed:
	new file:   TODO
	modified:   app.yaml
do renaming
	modified:   app.yaml
	modified:   database.py
	renamed:    handle_email.py -> client_mail_handler.py
	renamed:    stock_worker.py -> stock_task_handler.py
	renamed:    stock_updater.py -> stock_updater_main.py
refact query

add limit to client count
add limit to max subscribe count

	modified:   TODO
	modified:   client_mail_handler.py
	modified:   database.py
fix cron job bug
add replay for client's mail