Introduce ApplicationContextInitializer interface

Designed primarily for use in conjunction with web applications
to provide a convenient mechanism for configuring the container
programmatically.

ApplicationContextInitializer implementations are specified through the
new "contextInitializerClasses" servlet context parameter, then detected
and invoked by ContextLoader in its customizeContext() method.

In any case, the semantics of ApplicationContextInitializer's
initialize(ConfigurableApplicationContext) method require that
invocation occur *prior* to refreshing the application context.

ACI implementations may also implement Ordered/PriorityOrdered and
ContextLoader will sort instances appropriately prior to invocation.

Specifically, this new support provides a straightforward way to
programmatically access the container's Environment for the purpose
of adding, removing or otherwise manipulating PropertySource objects.

See Javadoc for further details.

Also note that ApplicationContextInitializer semantics overlap to
some degree with Servlet 3.0's notion of ServletContainerInitializer
classes. As Spring 3.1 development progresses, we'll likely see
these two come together and further influence one another.

org.springframework.core/src/main/java/org/springframework/core/env/PropertySource.java

@@ -63,6 +63,18 @@ public abstract class PropertySource<T> {
 		this.source = source;
 	}
 
+	/**
+	 * Create a new {@code PropertySource} with the given name and with a new {@code Object}
+	 * instance as the underlying source.
+	 * <p>Often useful in testing scenarios when creating
+	 * anonymous implementations that never query an actual source, but rather return
+	 * hard-coded values.
+	 */
+	@SuppressWarnings("unchecked")
+	public PropertySource(String name) {
+		this(name, (T) new Object());
+	}
+
 	/**
 	 * Return the name of this {@code PropertySource}
 	 */