Add transaction versioning, tx cancellation (#79)

migrations/1503907708756-drop-device-time.js

@@ -0,0 +1,14 @@
+var db = require('./db')
+
+exports.up = function (next) {
+  const sql = [
+    'alter table cash_in_txs drop column device_time',
+    'alter table cash_out_txs drop column device_time'
+  ]
+
+  db.multi(sql, next)
+}
+
+exports.down = function (next) {
+  next()
+}