Add support for varying the payment amount #53

Currently, the dollar amount on payments is recorded as the number of years times the membership fee (full or concession). There's times when this doesn't match what was actually paid:

1. Sometimes a member has included a donation alongside their membership payment.
2. Sometimes we've granted memberships for in-kind services.
3. We could conceivably offer an even further discounted membership in case of hardship.

It'd be nice if this could be reflected in the payment record. Some possible ways of implementing this are:

1. add a field for an extra amount to add to the membership amount. Negative values could be used to represent a discount.
2. add a field to record the total paid. If blank, calculate as before. Perhaps there could be some JavaScript to show what the other fields would calculate the total to be.