Single primary key for time_slot and receipt_item tables

* Modifies the receipt_item and time_slot to have a single "id" primary key, instead of a primary key across three columns. 
* Also adds a constraint to the time_slot table guaranteeing that hour will always be an hour of the day.

I will note that the current primary key constraint for these tables is valid, but the addition of the int "id" column makes integration with the javascript API much easier.