don't include comments in simplexml child tree.

SimpleXML comments are a weird concept. They are visible in var_dumps as
comment nodes, but you cannot access them through children(), nor are
they present in for example count().

This change will prevent comments from being processed at all. It means
we move away from 5.4 validity by one step, but moves us closer by a
couple more steps due to all kinds of funky bugs this fixes.
Merge branches 'simplexml-comments' and 'simplexml-xpath'