Invoice and line item styling

Updated the styling of the invoice and line item forms. Totals are now calculated and saved automatically, and amounts are calculated and displayed in USD currency format.

A callback was created for the LineItem model, so that quantity * unit_price calculates and updated the LineItem.price column.
New invoice fix

The user flow for creating new invoices is broken. A user could attempt to create a new invoice, and then click 'add line item' without first saving the invoice, causing an error. The 'add line item' link should save the invoice as-is, so that the user can either click save or add line item without running into an error. Ultimately, the 'add line item' button should allow the user to do so from the invoice page (via ajax). First the noJS version must be functional.