Exception is thrown when loading empty XML or HTML string
remove methods seqOr, flatMap, distr, _filter, _single and related crap
get rid of ugly and vile old secrets
move extractor into MatcherContext class, support for namespaces

MatcherContext is step towards more flexibility when Matcher class will
know only how to thread execution and match against any tree structure
and MatcherContext know how actual matching is done. So maybe one day...