KernelObjects: add JSObjectProxy>>value

return contents of the 'value' attribute if it exists
otherwise return the result of Object>>value
JSObjectProxy: add >>at:ifAbsent:
JSObjectProxyTest: add test for >>at:ifAbsent:
JSObjectProxy>>value: use >>at:ifAbsent:
JSObjectProxy: add unit tests for >>value
JSObjectProxy>>at:ifAbsent: take care of property which is set to 'undefined' (as suggested by Herby)
JSObjectProxyTest>>testAtIfAbsent: also test for property with 'undefined' value
Merge pull request #361 from mkroehnert/proxy

Add JSObjectProxy>>value