modify to support multiple grammars, add fmpxmllayout grammar

The approach taken was to convert the execute() method to route based on grammar and to move all of the parsing into grammar specific parsing methods. Also added an array property to validate that a supported grammar has been specified. fmxmlresult is the default, to support backward compatibility.
Still not possible to get there, since the setGrammar() method validates and throws. The comment is wrong because I moved the throw from the getURI method to the setGrammar method.