[BEANUTILS-428] Added FluentPropertyBeanIntrospector class.

This is a specialized BeanIntrospector implementation which also supports
properties whose setter method has a non-void return type. This is useful for
classes using a fluent API.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/beanutils/trunk@1540353 13f79535-47bb-0310-9956-ffa450edef68
[BEANUTILS-425][BEANUTILS-428] Added documentation.

The user guide now has an additional sub section describing the customization
of the bean introspection mechanism.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/beanutils/trunk@1540354 13f79535-47bb-0310-9956-ffa450edef68
Updated changes.xml for [BEANUTILS-428].

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/beanutils/trunk@1540355 13f79535-47bb-0310-9956-ffa450edef68