Multiple SimpleXML fixes.

#3048ed1 is 'questionable'. it breaks test hphp/test/zend/good/ext-simplexml/010.php but it fixes a lot of other screwed up behavior. The only reason why the test breaks is because comment nodes are no longer visible in var_dumps. On the other hand, it no longer shows up in ->children() (as it should), ->count(), empty root nodes, etc etc. Feel free to exclude it from your merge, however for me that change is crucial.

All other changes are either compatible with existing tests, or add their own tests that all pass. Feedback is of course always welcome.