Added PBKDF2PasswordEncoder.

- Also moved some logic into a new class, AbstractPasswordEncoder.
Both PBKDF2PasswordEncoder and the now-simplified
StandardPasswordEncoder extend AbstractPasswordEncoder.
 - Added tests for PBKDF2PasswordEncoder

Issue gh-2158

crypto/src/test/java/org/springframework/security/crypto/password/PBKDF2PasswordEncoderTests.java

@@ -0,0 +1,31 @@
+package org.springframework.security.crypto.password;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertFalse;
+import static org.junit.Assert.assertTrue;
+
+
+public class PBKDF2PasswordEncoderTests {
+    private PBKDF2PasswordEncoder encoder = new PBKDF2PasswordEncoder("secret");
+
+    @Test
+    public void matches() {
+        String result = encoder.encode("password");
+        assertFalse(result.equals("password"));
+        assertTrue(encoder.matches("password", result));
+    }
+
+    @Test
+    public void matchesLengthChecked() {
+        String result = encoder.encode("password");
+        assertFalse(encoder.matches("password", result.substring(0,result.length()-2)));
+    }
+
+    @Test
+    public void notMatches() {
+        String result = encoder.encode("password");
+        assertFalse(encoder.matches("bogus", result));
+    }
+
+}