Added cron expression validation method for CronSequenceGenerator

It's handy to know in advance whether or not expression that is
passed to CronSequenceGenerator or CronTrigger constructor would
not results in IllegalArgumentException. The only way to do it
now is to try\catch an instance creation but it's kinda ugly.

spring-context/src/test/java/org/springframework/scheduling/support/CronSequenceGeneratorTests.java

@@ -56,4 +56,18 @@ public class CronSequenceGeneratorTests {
 		new CronSequenceGenerator("*/-1 * * * * *").next(new Date(2012, 6, 1, 9, 0));
 	}
 
+	@Test
+	public void testValidExpression() {
+		assertTrue(CronSequenceGenerator.isValidExpression("0 */2 1-4 * * *"));
+	}
+
+	@Test
+	public void testNotValidExpression() {
+		assertFalse(CronSequenceGenerator.isValidExpression("0 */2 1-4 * * * *"));
+	}
+
+	@Test
+	public void testNullExpression() {
+		assertFalse(CronSequenceGenerator.isValidExpression(null));
+	}
 }