add PullPaymentCapable test

test/PullPaymentCapable.js

+contract('PullPaymentCapable', function(accounts) {
+
+  it("can't call asyncSend externally", function(done) {
+    var ppc = PullPaymentCapable.new();
+    assert.isUndefined(ppc.asyncSend);
+    done();
+  });
+
+  it("can record an async payment correctly", function(done) {
+    var ppce;
+    var AMOUNT = 1000;
+    return PullPaymentCapableExample.new()
+      .then(function(_ppce) {
+        ppce = _ppce;
+        ppce.callSend(accounts[0], AMOUNT)
+      })
+      .then(function() {
+        return ppce.payments(accounts[0]);
+      })
+      .then(function(paymentsToAccount0) {
+        assert.equal(paymentsToAccount0, AMOUNT);
+      })
+      .then(done);
+  });
+
+});

test/PullPaymentCapableExample.sol

+import '../contracts/PullPaymentCapable.sol';
+
+// Example class using PullPaymentCapable
+contract PullPaymentCapableExample is PullPaymentCapable {
+  // test helper function to call asyncSend
+  function callSend(address dest, uint amount) external {
+    asyncSend(dest, amount);
+  }
+}