Moved Escrows into an escrow subdirectory, improved docs. (#1430)

* Moved Escrows into an escrow subdirectory, improved docs.

* Fixed escrow mock.

* Fixed some more imports.

test/payment/escrow/Escrow.test.js

@@ -0,0 +1,11 @@
+const { shouldBehaveLikeEscrow } = require('./Escrow.behavior');
+
+const Escrow = artifacts.require('Escrow');
+
+contract('Escrow', function ([_, primary, ...otherAccounts]) {
+  beforeEach(async function () {
+    this.escrow = await Escrow.new({ from: primary });
+  });
+
+  shouldBehaveLikeEscrow(primary, otherAccounts);
+});