[membership_date2date] initial implementation

[membership_date2date] initial implementation

XXX: there are bugs in the dates of associate members
     when mixing several membership products

XML product can be made date to date with:
	<field name="membership_date2date">True</field>

XML invoice can be done like that:
	<function model="res.partner"
	 name="create_membership_invoice"
	 eval="(ref('res_partner_velorution_idf_42')
	 , ref('product_standard_member')
	 , {'amount':15, 'date_from':'2012-04-02'})" />
where date_from is %Y-%m-%d or None for today()

NOTE: membership currently needs this fix:
	diff -Nru0 old/membership.py new/membership.py
	--- old/membership.py   2012-12-21 05:59:41.930076247 +0100
	+++ new/membership.py   2012-12-21 05:59:06.338076588 +0100
	@@ -340 +340 @@
	-                    _membership_date, multi = 'membeship_start',
	+                    _membership_date, multi = 'membership_start',