swarm/services/chequebook: update chequebook for Solidity 4.3

- proper handling of failed send
- added fallback function with additional logging for top-ups
- added Solidity version pragma

swarm/services/chequebook/contract/chequebook.sol

@@ -1,4 +1,6 @@
-import "mortal";
+pragma solidity ^0.4.3;
+
+import "mortal.sol" as mortal;
 
 /// @title Chequebook for Ethereum micropayments
 /// @author Daniel A. Nagy <daniel@ethdev.com>
@@ -9,6 +11,14 @@ contract chequebook is mortal {
     /// @notice Overdraft event
     event Overdraft(address deadbeat);
 
+    /// @notice Deposit event
+    event Deposit(uint256 amount);
+
+    /// @notice Top up chequebook
+    function() payable {
+        Deposit(msg.value);
+    }
+
     /// @notice Cash cheque
     ///
     /// @param beneficiary beneficiary address
@@ -28,16 +38,18 @@ contract chequebook is mortal {
         // Attempt sending the difference between the cumulative amount on the cheque
         // and the cumulative amount on the last cashed cheque to beneficiary.
         if (amount - sent[beneficiary] >= this.balance) {
-            if (beneficiary.send(amount - sent[beneficiary])) {
-                // Upon success, update the cumulative amount.
-                sent[beneficiary] = amount;
+	    // update the cumulative amount before sending
+            sent[beneficiary] = amount;
+            if (!beneficiary.send(amount - sent[beneficiary])) {
+                // Upon failure to execute send, revert everything
+                throw;
             }
         } else {
             // Upon failure, punish owner for writing a bounced cheque.
             // owner.sendToDebtorsPrison();
             Overdraft(owner);
             // Compensate beneficiary.
-            suicide(beneficiary);
+            selfdestruct(beneficiary);
         }
     }
 }