added num_in_stock column to Inventory table, so we can show the user how many items are actually available before they attempt to make a rental.
please note that a critical assumption of this code is that there are currently NO items rented out, since we initialize num_in_stock to num_of_items.
To do this, the migration WILL destroy all entries in the Rental table.

Run a `rake db:migrate` to fully apply the changes.
num_in_stock properly set to num_of_items on creation, and views updated to show num_in_stock info
creating/deleting a rental now updates the num_in_stock counter in the corresponding inventory item. updates can still break things though, so don't do those.