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.
`$sfmresult` has to be defined before the control structure or in a default like this, or static will complain that it might not be set. You could alternatively set it to null before the control structure, or keep it here.