Refactor Environment and PropertySource

* Environment now extends PropertyResolver
* Environment no longer exposes resolver and sources
* PropertySource is String,Object instead of String,String
* PropertySource no longer assumes enumerability of property names
* Introduced EnumerablePropertySource for those that do have enumerable property names

org.springframework.expression/src/test/java/org/springframework/mock/env/MockPropertySource.java

@@ -36,7 +36,6 @@ import org.springframework.core.env.PropertySource;
  *
  * @author Chris Beams
  * @since 3.1
- * @see MockEnvironment
  */
 public class MockPropertySource extends PropertiesPropertySource {