An initial implementation of transactions.
Summary: This is functionally the audit log of the financial bits of the new hkccp, and should transcribe all transactions for a wallet -- credit & debit line items.
Test Plan: None
Reviewers: #hkccp_developers, machniak
Reviewed By: #hkccp_developers, machniak
Subscribers: machniak, #hkccp_developers
Differential Revision: https://git.kolab.org/D1102