Fix precision, scale of decimal columns

The columns were originally created with precision=8 scale=5, which does
not allow prices greater than 999.

This changes
* price to the same as line_item_price (precision=8 scale=2)
* ship_total to the same as adjustment (precision=10 scale=2)
* amount to precision=13 scale=5, which can fit the maximum allowable
  amount of either column at 5 decimal places of precision.
Merge pull request #34 from freerunningtechnologies/column_scale_fix

Fix precision, scale of decimal columns on cart_item