Parameterize exception type in RetryCallback

So RetryCallback<T, E extends Throwable> and the E parameter appears
in RetryOperations too, making it possible to call it with an unchecked
exception type in the parameter and not catch exceptions.

Users should beware: it's just syntactic sugar, and the actual runtime
type of the exception is never checked at runtime. So, for instance,
declaring a RetryCallback<Object,IllegalArgumentException> doesn't
mean that other Exceptions won't be retried, just that you won't be
able to explicitly throw them if they are checked.

A project using Spring Batch 2.2 was used to test that this works
with user code that uses a library compiled agains Spring Retry 1.0.

Fixes gh-6

src/main/java/org/springframework/retry/RetryCallback.java

@@ -23,7 +23,7 @@ package org.springframework.retry;
  * @author Rob Harrop
  * @author Dave Syer
  */
-public interface RetryCallback<T> {
+public interface RetryCallback<T, E extends Throwable> {
 
 	/**
 	 * Execute an operation with retry semantics. Operations should generally be
@@ -33,5 +33,5 @@ public interface RetryCallback<T> {
 	 * @return the result of the successful operation.
 	 * @throws Exception if processing fails
 	 */
-	T  doWithRetry(RetryContext context) throws Throwable;
+	T  doWithRetry(RetryContext context) throws E;
 }