Refactored Sha256Hash API:

- depracated constructors
- wrap methods to wrap given hash values in a new instance
- of/twiceOf methods to calculate hash values in a new instance
- hash/hashTwice to calculate hash values and return the raw bytes

tools/src/main/java/org/bitcoinj/tools/BuildCheckpoints.java

@@ -116,7 +116,7 @@ public class BuildCheckpoints {
             buffer.position(0);
         }
         dataOutputStream.close();
-        Sha256Hash checkpointsHash = new Sha256Hash(digest.digest());
+        Sha256Hash checkpointsHash = Sha256Hash.wrap(digest.digest());
         System.out.println("Hash of checkpoints data is " + checkpointsHash);
         digestOutputStream.close();
         fileOutputStream.close();