wrote tests for deleteMatchingStatement and adapt it

From now on, it checks if SPO are valid, that means:
- is model really available
- is $subject an URI or null
- is $predicate an URI or null
- add function valideObject to have a more clear way
  to check what type the $object is, array or string or
  null (now its possible to handle a given uri-string as
  as uri, which is usually represented by an array)