Add M:M for economic events creating commitments

This relates to issue #244 (closed) , for some new experience with distributing a budget for work.

I made the relationship where an economic event implies / creates a commitment into a many-to-many. I named it Reciprocity. I'd love some feedback on the name, I wasn't able to come up with exactly the right thing. Note it is the same structure as Fulfillment, but has a different real world meaning and sequence of work flow. I'm going to implement it soon for FairCoop, so it would be great to have some VF consensus on it first. :)

I added a reference to an agreement to both Fulfillment and Reciprocity, as I think it may be wanted at that granularity.

I also added DistributionAgreement as a subclass of Agreement, although in some ways it is a kind of ExchangeAgreement. But different user thinking and structure. Anyway, I'm happy removing it if people don't like it for any reason, let me know here. I don't need it for this round of FairCoop work.