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

* Moved Escrows into an escrow subdirectory, improved docs.

* Fixed escrow mock.

* Fixed some more imports.

(cherry picked from commit f3df2dab3d)

contracts/payment/escrow/ConditionalEscrow.sol

@@ -0,0 +1,22 @@
+pragma solidity ^0.4.24;
+
+import "./Escrow.sol";
+
+/**
+ * @title ConditionalEscrow
+ * @dev Base abstract escrow to only allow withdrawal if a condition is met.
+ * @dev Intended usage: See Escrow.sol. Same usage guidelines apply here.
+ */
+contract ConditionalEscrow is Escrow {
+  /**
+  * @dev Returns whether an address is allowed to withdraw their funds. To be
+  * implemented by derived contracts.
+  * @param payee The destination address of the funds.
+  */
+  function withdrawalAllowed(address payee) public view returns (bool);
+
+  function withdraw(address payee) public {
+    require(withdrawalAllowed(payee));
+    super.withdraw(payee);
+  }
+}