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.
Class constants would be defined like this at the top of the class:

```
    /**
     * fm/xml grammars
     */
    const FMRESULTSET_GRAMMAR = 'fmresultset';
    const FMPXMLLAYOUT_GRAMMAR = 'FMPXMLLAYOUT';
```