BeanConverter

Attached to this issue you'll find the BeanConverter I've coded
along the lines of the ReflectionConverter. bean.zip containes classes
that should go in com.thoughtworks.xstream.converters, whilst beanTest
contains the correspondent unit tests.

The BeanConverter will accept every class with a default construtor
and marshal/unmarshal every simple property that has both a getter and
a setter. Since it works exclusively on public methods, it should work without problems in applets too.

During the coding I've removed the management of implicit collections,
since I don't need it and it made things more complicated.

Since the bean converter accepts every class with a default construtor, it's better to make it the default converter.