Add period support to membership functionality

Background:

Current membership functionality of OpenERP let users define membership products with required fields: "Date from" and "Date to" that make it possible to define for example "X membership that is valid form 01/01/2011 to 01/01/2012" then a customer can purchase the X membership and their membership will be valid till 01/01/2012.

If user wants to sell membership for a specific period of time, for example "Y membership that is valid for 6 months", user can create a membership product with fields: "Date from = 01/07/2011" and "Date to = 31/12/2011" and sell it to a customer; when the next customer wants to buy membership a week later, the "Y membership product" can't be used as it is valid till "31/12/2011" and user must create an new membership product with fields: "Date from = 07/07/2011" and "Date to = 06/01/2012".

So with current implementation user must create new membership product everyday he has a new customer, and if user is selling membership for different periods, say 3 months, 6 months, 12 months, then user must create 3 new membership product per day!

Requirements:

* Ability to choose days, weeks, months and years as period of memberships
* Ability to choose the invoice day as "Start membership date"
* Ability to choose the payment day as "Start membership date"

Possible solutions:

1) Improve membership addon module so it could allow defining memberships with periods.

2) Create a new membership_period addon module so it could add ability to define membership products with periods.

3) Make it possible by only changing OpenERP settings, e.g. user could adjust "Start membership date" and "Stop membership date" when selling a new membership