1. this makes the code able to run on both Python 2 and 3 (previous version only support 2);
2. Use placeholder to prepare SQL INSERT statement. This is much more proper than using .format to prepare the statement. One obvious advantage is it can handle single/double quotations marks very perfectly. The previou version will fail if there is single quotation mark in the values that I'm going to insert.
This makes this app much more meaningful. For this feature, a new database table is added. More complicated logics are also needed, for example, we use hash(user ID + timestamp + note contant) as the unique id of each note (it's necessary when we try to delete a note)
