A dynamic bill creation page

[kelvinst]

The bill creation will be done in a simple step, with no complexity, and from there, the user will be able to edit and configure bill items and members. Here are the steps to get this done!

• [x] The bill basic information, fields: creator, payer, members (a multiple select field), total_amount
• [x] In edit, update the value of total_amount in the main item, if they were sent
• [x] Put bill items on edit and show bill page
• [x] Put bill members on edit and show bill page, when the bill have just one item
• [x] Put bill members on edit, new and show bill item page
• [x] When there is more than one bill item
  • [x] Hide the members and total_amount fields
  • [x] Show a total amount with the sum of the items at the end of the panel

[kelvinst]

Depends on #5

[kelvinst]

I was thinking: maybe we can create the bill like a wizard kind of thing, like asking the basic information in the first page, and opening its show page after it, with links to add items from there.

It's easier to implement and easy to use as well.

I'm not discarding a very dynamic page. It's just that for now, maybe a simple page can do the work.

[kelvinst]

Work in progress on the kill-bill branch

[kelvinst]

@aramisf you can review #17 now!