Support having members share costs only beginning from a certain date

[cb109]

Take e.g. a shared flat: People come and go. A new flatmate should not be responsible for old purchases. Also we should not be forced to create a new Collective everytime the member number/constellation changes (which is currently pretty much the only effective way to deal with this).

This could probably be implemented by adding a join_date to each Membership and calculating each member's share of the Purchases/Liquidations starting from that date only, so that someone joining later will effectively have to pay only what he made use of.

[cb109]

Actually it would also be nice to support includes/excludes per Purchase

[cb109]

If we have a joined-date we also need an quit-date. And we may need to support multiple memberships of one User to a Collective to allow rejoining.

[cb109]

We'd also have to think about how to display transfers etc. by Users that are no longer (or not currently) a member.

[cb109]

Has some overlap with https://github.com/cb109/payshare/issues/63