Minimum funds for payer to contribute

[patriacaelum]

There may be a minimum amount of funds a payer must pay if they choose to contribute. This is to help ensure payers will not play a long waiting game of trying to contribute as little as possible to get a proposal running.

• Add a parameter that defines the minimum payer contribution
• In the ProposalInverter.pay() method, the payer's contribution should be checked to be greater than or equal to the minimum payer contribution