#51 -> fixed request for invoiceProduct

Client has been taught to recognize JSON containing
object of InvoiceProduct type

Signed-off-by: kornicameister <kornicameister@gmail.com>
#9 -> refrectored

Signed-off-by: kornicameister <kornicameister@gmail.com>
#51 create -> invoice

invoices are now created
in very good manner
what is left is to provide server side allocation algorith
that will get products and place them in some good units

Signed-off-by: kornicameister <kornicameister@gmail.com>
#9 -> db

updated database model

Signed-off-by: kornicameister <kornicameister@gmail.com>
#51 -> almost

I can send valid list of associated invoices and products
and launch algorithm to match product to unit in most basic
form, but I have the bug that is related to modyfying @Formula in models

Signed-off-by: kornicameister <kornicameister@gmail.com>
#9 -> refractored

Signed-off-by: kornicameister <kornicameister@gmail.com>
fix #51, products are allocated really nicely