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

* Moved Escrows into an escrow subdirectory, improved docs.

* Fixed escrow mock.

* Fixed some more imports.

contracts/payment/escrow/RefundEscrow.sol

@@ -0,0 +1,91 @@
+pragma solidity ^0.4.24;
+
+import "./ConditionalEscrow.sol";
+
+/**
+ * @title RefundEscrow
+ * @dev Escrow that holds funds for a beneficiary, deposited from multiple
+ * parties.
+ * @dev Intended usage: See Escrow.sol. Same usage guidelines apply here.
+ * @dev The primary account (that is, the contract that instantiates this
+ * contract) may deposit, close the deposit period, and allow for either
+ * withdrawal by the beneficiary, or refunds to the depositors. All interactions
+ * with RefundEscrow will be made through the primary contract. See the
+ * RefundableCrowdsale contract for an example of RefundEscrow’s use.
+ */
+contract RefundEscrow is ConditionalEscrow {
+  enum State { Active, Refunding, Closed }
+
+  event RefundsClosed();
+  event RefundsEnabled();
+
+  State private _state;
+  address private _beneficiary;
+
+  /**
+   * @dev Constructor.
+   * @param beneficiary The beneficiary of the deposits.
+   */
+  constructor(address beneficiary) public {
+    require(beneficiary != address(0));
+    _beneficiary = beneficiary;
+    _state = State.Active;
+  }
+
+  /**
+   * @return the current state of the escrow.
+   */
+  function state() public view returns (State) {
+    return _state;
+  }
+
+  /**
+   * @return the beneficiary of the escrow.
+   */
+  function beneficiary() public view returns (address) {
+    return _beneficiary;
+  }
+
+  /**
+   * @dev Stores funds that may later be refunded.
+   * @param refundee The address funds will be sent to if a refund occurs.
+   */
+  function deposit(address refundee) public payable {
+    require(_state == State.Active);
+    super.deposit(refundee);
+  }
+
+  /**
+   * @dev Allows for the beneficiary to withdraw their funds, rejecting
+   * further deposits.
+   */
+  function close() public onlyPrimary {
+    require(_state == State.Active);
+    _state = State.Closed;
+    emit RefundsClosed();
+  }
+
+  /**
+   * @dev Allows for refunds to take place, rejecting further deposits.
+   */
+  function enableRefunds() public onlyPrimary {
+    require(_state == State.Active);
+    _state = State.Refunding;
+    emit RefundsEnabled();
+  }
+
+  /**
+   * @dev Withdraws the beneficiary's funds.
+   */
+  function beneficiaryWithdraw() public {
+    require(_state == State.Closed);
+    _beneficiary.transfer(address(this).balance);
+  }
+
+  /**
+   * @dev Returns whether refundees can withdraw their deposits (be refunded).
+   */
+  function withdrawalAllowed(address payee) public view returns (bool) {
+    return _state == State.Refunding;
+  }
+}