add poex example

contracts/examples/ProofOfExistence.sol

+// Proof of Existence contract, version 3
+contract ProofOfExistence3 {
+
+  mapping (bytes32 => bool) proofs;
+
+  // store a proof of existence in the contract state
+  function storeProof(bytes32 proof) {
+    proofs[proof] = true;
+  }
+
+  // calculate and store the proof for a document
+  function notarize(string document) {
+    var proof = calculateProof(document);
+    storeProof(proof);
+  }
+
+  // helper function to get a document's sha256
+  function calculateProof(string document) returns (bytes32) {
+    return sha256(document);
+  }
+
+  // check if a document has been notarized
+  function checkDocument(string document) returns (bool) {
+    var proof = calculateProof(document);
+    return hasProof(proof);
+  }
+
+  // returns true if proof is stored
+  function hasProof(bytes32 proof) returns (bool) {
+    return proofs[proof];
+  }
+}