[#7] Donate per use

[joraff]

As a member,
I want to attach a small donation to my "rental" of equipment,
So that we may work toward the common goal of increasing the group's holdings.

[rstackhouse]

I'm trying to keep issues in here to a user story format until we decompose them into tasks. I am trying to defer detail until "the last responsible moment" here. I'm also trying to avoid over-design/specification.