invoice editing: make total amount calculations

[jzarate]

make the final result calculation (cost, taxes, discounts).

also , make the invoice form displays new totals as the user edits new unitary cost or quantity or discount values.

[jzarate]

r191

[jzarate]

r192 added even listening for dom-created nodes

r193 added 2 decimal rounding

[jzarate]

r194 subtotal field

Enrique, I used the following model for the subtotals calculation. I need confirmation on this:

• subtotal: the gross sum of the total amount of each item, discounts of the item aplied, taxes excluded.
  
• taxes: the overall sum of all the items taxes, whatever they be. That field is label with a generic "taxes", since each item can have different taxes types.
  
• discount: the global discount defined in the field "customer discount", applied to the gross sum
  
• totals are calculated as this:
  

subtotal + taxes - discount

[jzarate]

r202 javascript refactoring

[jzarate]

r203 corrected small bug from r202

[jzarate]

r272 adpated to the new fields related to "totals" in the invoice model. these fields are:

• base : the base amount (sum qty*u_price for each item)
• discount : the total discount (sum the discounts -not in percentage, but in real value- that are to be applied for each item)
• taxes: the total taxes (sum over the taxes to be applied for each item -not in percentage, but in real value)
• net: the price after discount (summed over every item: base amount - discount)
• gross: the final price (base - discount + taxes).