More Versatile Boolean conversion options

This patch adds new boolean conversion options that can be easily enabled via code like this:

class Perms {
boolean readable = true;
boolean writable = false;
}

xstream.registerConverter(BooleanConverter.TRUE_FALSE); // the default, same as before

<perms>
  <readable>true</readable>
  <writable>false</writable>
</perms>

xstream.registerConverter(BooleanConverter.YES_NO);

<perms>
  <readable>yes</readable>
  <writable>no</writable>
</perms>

xstream.registerConverter(BooleanConverter.BINARY);

<perms>
  <readable>1</readable>
  <writable>0</writable>
</perms>

xstream.registerConverter(BooleanConverter.EMPTY_ELEMENT);

<perms>
  <readable/>
</perms>

Note that the EMPTY_ELEMENT BooleanConverter requires the XSTR-308 (http://jira.codehaus.org/browse/XSTR-308) patch that allows a converter to say weather or not a tag should be written for the value.