* Support a third paramater to Parser::setHook() that can contain the object

* Support a third paramater to Parser::setHook() that can contain the object
  that contains the callback function to use
* Add a hook to Parser::getVariableValue() (ParserGetVariableValueVarCache')
  that allows overriding of the variable cache
* Add a hook to Parser::getVariableValue() (ParserGetVariableValueTs) that
  allows overriding of the timestamp
* Add a hook to Parser::getVariableValue() (ParserGetVariableValueRevid) that
  allows overriding of the revision id
* Changed Parser::getVariableValue() to use timestamps in such a way that they
  can be overriden with the ParserGetVariableValueTs hook