duration_unit_price is calculate with the discount, but when we generate invoice lines, the discount of line price seems to be also calculate, so using real_unit_price could resolve this problem
- Change the coeff display in float number with a precision of 2. 
- Calculate unit_price with the coeff in invoice line was missed. Now it's working. 
- Modify the map of coeff for more realistic value for my use.
Add the possibility to make invoices when state is done.
Add the possibility to make invoices when state is done.
Update - Files of openerp-rent in production
Add the possibility to have free product

Necessary to have the possibility to put product.rent_price = 0 for
service who don't have price, and for make notice lines.
Merge pull request #1 from jeffisnotdead/master

free product