This introduces a class for nicer wrapping and easier testing. Some
functions were changed to provide nicer APIs (no throwing around of
unescaped regexps) and to simplify things (hopefully).

The refactoring isn't completed yet, so this will break the subscription
system.

The goal is to move as much subscription related stuff to this class as
possible. Currently there is some code in lib/exe/indexer.php and maybe
elsewhere (common.php?). Additionally everything should be covered by
tests. A few tests are included here already.